I believe it was clearing out the old snapshots.
For this you'll need a tool called Onyx. It's free software that has existed for MacOS for many many years, almost as far back as OS X (first version was for MacOS 10.2).
https://www.titanium-software.fr/en/onyx.html
- This step alone did not fix the problem but possibly contributed to it. I removed FindMy from my Desktop Widgets since it was a FindMyWidget file that was throwing an error when TM would fail as you can see a few posts ago.
- Open Time Machine settings, click Options, change Backup Frequency to Manually.
- Open Onyx and enter your password.
- Go to the Maintenance Tab. Click Restore Defaults to make sure the default options are enabled. Click Run Tasks. It will close all open apps, run the tasks, then reboot your system.
- Open Onyx and enter your password.
- Go to the Utilities Tab. Click APFS Snapshots. Click Delete APFS Snapshots.
- Close Onyx.
- Open Time Machine Settings, click Options, change Backup Frequency to Automatically Every Hour.
For me these steps stopped the error and has worked for over 12 hours now. I verified this morning that backups did occur and complete successfully while the system was "asleep". My sleep settings are set at defaults so that the system does sleep when the screen is off and sleeps hard drives when possible. Changing those settings did not resolve my error anyway so back to default I went.