Recurring 'couldn't complete backup' issue with TimeMachine on Mac mini

TimeMachine is giving me problems. After erasing and then reformatting the backup disk, TimeMachine runs maybe 25 or 26 times then throws a "couldn't complete the backup" flag. It retries each hour after that and identifies a different file each time as the offering reason for terminating. Example files it has problems with are:

1) %@

2) /.file

3) /private/var/MobileSoftwareUpdate

4) /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Mac Mini/2025-01-12-100616/Data/mnt

I have excluded both "/Applications" and "/Library" from the backup as well as multiple folders in the system folder (I have tried various combos of folders but nothing works and I can't access the . or private files from TimeMachine's possible exclude list, and have no idea where it thinks %@ is located). BTW, when this happens the backup disk has over 2TB available.

I've run out of ideas on what to try, and suggestions are welcome.

My info: a Mac Mini 2024 M4 Pro, 2 TB drive, running 15.2. The backup disk is a Micron Crucial 4TB portable SSD using an APFS format with a GUID Partition Map (I have tried other formats but with same result).

[Re-Titled by Moderator]