Well! I installed iTunes 12.8.1 today on Sierra, and lost contact will all my iDevices via Wi-Fi or USB. I rebooted and tried again, no joy. I then use iMazing to reset the pairing records. I was then able to connect each device to iTunes via USB and they stayed connected via Wi-Fi when I disconnected the USB. I then shut down iTunes, which took an inordinately long time. When I restarted iTunes, none of the Wi-Fi connections are there and it will not recognize them via USB when plugged in. I expect if I reset the pairing records again that should work, but what a pain. I’m going to see if I can go back to the previous iTunes.
iTunes 12.8.1 was pulled from the App Store today. I suspect it will either be reposted or replaced by a new version shortly.
-Al-
Since it appears to be exceedingly difficult (impossible?) to go back a version, I do hope they get a replacement out quickly. Eventually after reseting the pairing records again, I was able to 3 of my 4 devices connected, but have zero confidence that those connections will continue to work.
@Simon, you either missed, or misstated whether or not you applied my own magic sauce for what was the identical error messaging and subsequent inability to sync, either via USB or WiFi, and yet, I, too, did eventually notice I’d maintained the ability to do backups.
Again, in my case, after having identified a confirmed bad cable (testing with other devices), and replacing it with a confirmed good cable (again, testing with other devices), I, too, could still not sync via USB or WiFi, but only on that one device…
(Note: I’m not saying a bad cable triggered your identical issue; but it may or may not have been the cause of my identical issue.)
Magic Sauce: I attached the Unsyncable Folly Frown (ask your grandmother) to another Mac to which it had never been tethered; opened iTunes; answered yes to the ‘trust’ message ("Do you want to allow blah blah blah… ); then also on the iOS device said yes to the ‘trust?’ message (requires passcode); verified it was recognized by iTunes (in this case, 12.x.x; whereas my host Mac is locked on the 12.6/x track just like you); disconnected without doing any sync or backup; reconnected to my original Mac with the known-good cable; and it immediately both backed up and synced without further error; it again subsequently synced via WiFi; and has synced via WiFi nightly ever since.
F
HTH
This is also an excellent suggestion; I seem to recall this around Mountain Lion or El Cap, as well, and recall more than one time purging both Music and Ringtones; I don’t recall for sure if I once had to purge Photos, but I think I did; and this, as stated, odious, time-consuming (albeit overnight) approach (assuming you have a large personal music library) cleared the logjams.
It suppose it could happen with any category; TV/Movies/Books, whatever; but as long as you have everything otherwise backed up, it’s kind of an in-between option to just nuking it and restoring from backup.
Once again, many thanks kind folks for all the great suggestions. I’ll see if I find some time over the weekend to work on it.
It appears going El Cap -> HS cannot be expected to fix it. It also appears that I might be able to install the older “special” iTunes version (the one with the apps) and although that might be compatible with iOS 12 now, it’s unclear how sustainable that path is. Plus of course, there is so far no clear indication that that will actually solve the syncing issue. So I suppose I’ll try that last.
iMazing is what I’d like to try next.
Actually, before that I will definitely try @frederico’s secret sauce. I did indeed miss that magic ingredient in your recipe. Should be simple to go back and test. Just so I understand properly, when I hook up my wife’s SE to my MBP and have both devices trust each other, I can safely stop anything else from happening and disconnect, right? Considering the state her iPhone is in, she is vey concerned about losing stuff and from what I remember way back when when you sync an iPhone to a different Mac (or I should probably say, to a different iTunes account), it wipes a whole bunch of stuff. I just want to be sure that doesn’t happen before I yank the cord.
Thanks also, @seth, for that suggestion. That seems very straightforward to try out. I’m not sure, however, the sync is even getting that far that selecting what to sync has any bearing. You see, when I connect her SE to her Mac, iTunes at the top will immediately display something like “step 1 preparing for backup” but it then gets stuck there for a minute or so before it exits and throws up the error message. We’ve at this point not even come close to syncing. We’ve not even backed up yet only after which the device would sync. At least that’s how it used to work for her and it still does for me with my old 6 (iOS 12) and MBP (HS). Now, in principle I guess it’s possible the backup has completed and it’s just the GUI feedback that’s lagging, and the reason we’re actually aborting is related to sync issues. But in such a case, I would expect to see a record of the completed backup either in iTunes or the file system. But there’s nothing there leading me to believe that the backing up part is the problem and we haven’t reached sync yet. But now that I write that I realize that when I was able to get a new backup to finish (although iTunes still gave the same error, upon restarting iTunes the new backup showed and I see it in Finder too), it was because I had selected to do just the backup. That OTOH would indicate that selecting sync (which first invokes backup) does cause the process to fail in a different way that just selecting to back up alone. I guess after all this rambling, the conclusion is that I probably should try it anyway. I’ll get back to you with results.
And finally, any idea what’s up with 12.8.1 being pulled? I feel super great now about having installed on her system an iTunes Apple no longer has faith in - as if the woman doesn’t have enough iTunes/iPhone grief already. 
I don’t know all the reasons why it was pulled but I have had tons of grief and a couple of hours of time involved simply attempting to get my 4 devices connected. I have succeeded with 3 of them but the fourth is being problematic. I was going to open a ticket with Apple but did not once I saw it was pulled. Hopefully the fix will also resolve my issues.
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Correct.
And if iTunes on the standard host Mac can see the trouble child, regardless of connection via USB or WiFi, you should be able to go in to each category and selectively deselect Music, etc, then click ‘Apply’, without first initiating a sync.
Or am I misunderstanding you? I get that the sync fails immediately upon mounting it via USB, but are you then unable to select the iphone from the ios devices “button/popdown”, where you can then view the basic info page to perform manual backups/sync/choose various settings/update/restore/etc?
If you can see that screen, select Music (or whatever) and deselect something/everything, then click ‘Apply’.
This is the suggestion that might help reset whatever is breaking your sync abilities.
I guess I’m also confused as to how deep the loss of data that exists in the iPhone, but isn’t contained in the most recent backup really is; are we talking priceless photos, because your wife refuses to use even Photo Stream? Notes, not syncing with iCloud? Contacts? POP email? SMS?
Most of those I just listed can be sideloaded in one way or another; saved games and recent diagnostics are among the few things I can think of that would be difficult to salvage without a recent backup.
If you could be more clear what it is you fear is going to be forever lost, assuming you cannot make a reliable backup manually sans a successful automatic sync (which first includes a backup) right now, and it turns out that every single historical backup on your Time Machine are also hopelessly corrupt, it might help to just work to protect certain dealbreaker categories, and just nuke the darn phone.
So here’s a brief update. Not good.
1.) Tried deactivating certain types of media to get syncing to complete. Didn’t actually do music since it claimed it would delete all music. Considering putting those 50 GB back onto the iPhone (if it even can) would take hours, I just wasn’t ready to go that route. Anyway, with other stuff deactivated, still no change.
2.) Tried connecting her iPhone to my Mac. Both devices connected. Was even able to make a backup of her iPhone on my Mac. Upon reconnecting to her (in the meantime restarted) MBP no change.
3.) Downloaded iMazing. Found the reset sync option (somewhat odd to hide that in preferences). Did not do anything. Despite rebooting etc.
4.) Downloaded iTunes 12.6.5 and installed that on her Mac. Upon trying to launch that, got error message “iTunes Library.itl cannot be read because it was created by a newer version of iTunes. Would you like to download a new version of iTunes?” If you click yes you’re then taken to MAS which shows (despite several refreshes) no updates available.
Arrrrghh. This is like ****ing Windows. I’m foaming at Apple right now. Could you please stop breaking people’s **** for no good reason? 
Given that Apple has been deprecating support of IOS devices through iTunes, why not see if iMazing will be an adequate substitute for local backup and sync of IOS devices?
Currently, as far as my devices are concerned, they backup to iCloud, but I also maintain local backups via iMazing.
Just delete the iTunes Library.itl file and it will be recreated in the required format.
I use iTunes backups when I want to do a manual backup, like before a new iOS release. I use iMazing all the time because they are automated, I have it backup each of my devices every night after midnight. I also use iCloud backups. Just cannot have too many backups.
I finally got around to trying that. The old iTunes (12.6.5) then started up but it still refused to sync. Just the same as before.
So I downloaded the latest iTunes version form apple.com and put the original .itl file back. iTunes launches and seems to show all her music again (after deleting the itl file the old iTunes did not). But with the 12.8 iTunes it still won’t sync. It appears to back up (still throws an error saying it failed though), but sync will not happen.
Any other ideas for what we could try? I just cannot imagine that she should basically no longer be able to use iTunes with her perfectly working iOS 12 iPhone just because of, well, who knows what. 
Apple’s desire to force all iPhone users to use iClod for syncing?
While a lot of people bitched that iTunes was too “bloated”, I found it just about perfect except for the gray-on-gray icons that replaced the proper color icons. What Apple should do is what they did with iBooks: make standalone iOS App buying and syncing applications for Mac OS & Windows. Return iTunes to its SoundJam roots and have it just handle music. I’m slowly moving over to iMazing2 in lieu of the current iTunes.
Other ideas. I gave up. I made no OS or iOS changes between Oct & Dec and iTunes refused to sync or backup. I bought an app mentioned earlier in this thread and backed up that way. What is needed are not Apple music apps - maybe not Apple hardware too.
So in the meantime we’ve installed both the recent iTunes 12.8.2 update on her Mac along with the iOS 12.1.3 update on her iPhone SE. Both devices were rebooted. Although iTunes gives an error at the end of the backup process, it does show a recent backup has been saved to disk. So at least that “worked”.
However, syncing still does not work. It won’t even start to sync. Regardless if we try to do it over USB or if we try over wifi (tried both using the sync button in iTunes and the sync button on her iPhone). No matter what we try, sync just won’t start let alone complete. After a while iTunes will simply display an error message “The iPhone could not be synced because the sync session failed to start.” The iPhone will not update its display at all.
Any other suggestion at all what she could attempt to try to get her iTunes to sync with her iPhone again?
Every time I have sync problem between iPhone & iTunes on the Mac (which is frequently) I use iMazing to Restart Mobile Device Services, that works most of the time. Occasionally I need to use iMazing to Remove All Pairing records. I always am able to get sync going again.
I have been wondering why sync services does not seem to be restarted with a macOS restart, and have wondered if a power off, power on restart would make a difference but have not tried it.
I don’t know of any other tool than iMazing that gives access to those two items, but there may be something else.
Since iTunes is basically useless anymore for moving new tunes to the iOS Music App, can anyone recommend some other iOS player for the MP3s I have owned since the late 90s? Anyway to move playlists back and forth? (I do own iMazing.)
We tried iMazing, but as detailed above it did absolutely nothing for us.
The saga continues.
My wife just got a brand new 128 GB iPhone SE from Apple’s clearance section for $299. So in order for her to migrate to the new iPhone she first needs to get her old iPhone to sync properly with her Mac.
We tried everything noted above. In the end every single item was switched off and still the sync would not complete. I caved and upgraded her Mac from El Cap to HS since that I believe is the last macOS her mid 2010 MBP will run. With everything updated, the latest iTunes in place, and both Mac and iPhone rebooted, we tried again. No sync. Neither over wifi nor over USB. On the third of fourth retry we got it to sync contacts and calendar, but that was it. From then on nothing. We used iMazing to reset the syncing, but that didn’t help either.
So I finally convinced her to at leaat try backing up to iCloud. We did that and it worked. So we popped the SIM out of her old iPhone and into the new. Had to first set up the new iPhone as new phone so we could update to the latest and greatest iOS which her old iPhone was already on (turns out that clearance iPhone had been manufactured in March 2017 and been sitting on a shelf since). Then in iTunes we chose to set up her new iPhone from the last backup of her old iPhone. That did work, her settings were all there. However, none of her other apps were on the new iPhone, her music, photos, etc. is not on it either.
We can get the Mac to sync to her new iPhone, but since the old iPhone is not syncing to her Mac, the Mac doesn’t have the latest version of all of that. Unless we first get the old iPhone to sync properly to her Mac, how can we get any newer content onto her new iPhone?
I’d really appreciate any advice you might have. I’m really out of ideas at this point.