iPhone backup to Mac fails IOS 15.1 MacOS Monterey 12.0.1

Oh, how I hate problems that only happen some of the time!

More data points:

• Wife's phone backs up OK to her Late 2014 Mac Mini (Monterey 12.0.1 and IOS 15.1, iPhone 8, so essentially the same configuration that failed for me, except for different amounts of storage, music loaded into phone, etc.).
• My phone backed up OK last night to MacBook still running Big Sur

I updated the MacBook to Monterey last night. FWIW, this morning the "Back Up Now" button is greyed out, so not sure if it is trying to tell me it's too soon to back up again, or that nothing has changed on the phone so no back up needed, or if it is a problem.

This morning, my phone backs up OK to my Mac Mini, which had not worked for at least a couple weeks. Maybe backing up to a different Mac changed something that made it work again? Who knows.. (Before I retired as a software engineer, I would go into the source code to diagnose these things -- not possible in this situation, obviously. Flakey things like this were always the hardest ones to track down. Working at the level of superstition here, and I hate it!)

I want to leave this open for a while to see if it recurs.

Late 2014 Mac Mini -- oldest one supported on Monterey.

Yes, using iCloud to sync things like Contacts, Calendar, and Notes (but not Photos or Music). But this was all working (although a bit erratically) until about two weeks ago. Not sure if the failure corresponded to installing Monterey, or IOS 15, or something else. Sadly, Apple moving the iPhone/iPad syncing code from iTunes to Finder did NOT improve the code. But at least it was working most of the time, and, come to think of it, seemed to improve a bit with later versions of Big Sur. Now I can't get it to work at all. Will have to try tomorrow to see if wife's is working -- same phone, Mac Mini, same s/w versions, but different amounts of memory, different USB hub.