MacOS - Time Machine error ("files unavailable")

Been struggling with this problem for awhile now -- error reads:
"Time Machine did not finish backing up because some files were unavailable. Backups will resume when your Mac is unlocked."
There's various 'rebuild' suggestions from Apple, but the following fixed it for me
1.  On your Mac, open System Settings; goto "Lock Screen", see if "Require password after screen saver begins..." is greyed out. If so, then continue.
2.  On your Mac, launch the iPhone Mirroring App.
3.  Open "Settings" from its menu.
4.  Change from "Automatically Authenticate" to "Ask every time".
5.  Close the dialog box & Quit the iPhone Mirroring App.
6.  Go back to System Settings & check the Lock Screen settings - the "Require password after screen saver begins..." should no longer be greyed out. Try setting it to something other than "Immediate" for now.
Done (maybe do a reboot now to be sure)
Overall, I think that the iPhone Mirroring App's automatic authentication is the root cause of this TM error.
-hh