The Mac App Store shows I have one update to install. When I open it, I am prompted to log in. When I do, it appears to accept my password, and then I get an error dialog that says:
"An unexpected error occurred while signing in.
The data couldn’t be read because it isn’t in the correct format."
Closing and reopening the Mac App Store didn’t change anything.
I get a different error dialog on my MacPro running Mojave–but the same result. It will not accept my password and let me download an update. I’ve seen this happen before and it can take a few days to start working again.
When I encounter things like this on older systems, my default assumption these days is an expired security certificate. I realize that may not always be a useful answer, especially if the mechanism for updating certificates is not always obvious or even available to end users.