Just wondering if anyone else has seen this.
I closed all running apps and preceded with the 10.14.6 install today which, of course, restarted the iMac (Retina 4K, 21.5-inch, Late 2015). A bit later when I opened Calendar none of my calendars showed up. I have iCloud set up to sync the usual Apple apps including Photos to my herd of Apple devices so I checked Calendar on my iPhone and it was fine.
Everything else seems to have been unaffected.
I checked iCloud just to be sure the data was there, and all was fine through the browser interface.
In Calendars I tried to use Calendar > Accounts… and Calendar > Add Account… to force it to see the iCloud calendars and sync them, but was told I was already signed in to iCloud. I tried various dope slaps such as restarting the app and changing the View back and forth between Full Screen and Window, but no luck.
Finally I went to System Preferences > iCloud and signed out of iCloud on the iMac. Of course this meant turning off all the syncing of app data and reassuring the software that I really did want to turn off Find My Mac. Then I just signed into iCloud again. After it finished re-doing everything it had undone, I opened Calendar and all my calendars magically synced again. Problem solved, but what the heck caused it in the first place, and should there be a cautionary tale in here somewhere?