Tahoe Time Machine failing backup

I get the following error dialogue (when backing up my newish MacBook Pro) about once per reboot. Rebooting seems to solve the problem for about 12-18 hours and then it shows up again. To the best of my knowledge, I do not use any Game Center stuff. I tried deleting the Documents folder referenced and this did not help. And I don't believe my Mac was locked at the time. I believe this started after updating to 26.1, not sure how soon after the update that the problem started, but never saw it prior to the update.