Time Machine may not be able to copy a file for some reason.
Jump to end of answer if you want to know the solution without reading the details.
I had my Time Machine backup to a WD MyCloud NAS fail similarly. I tried to find the cause using
log show --style syslog --last 5m --predicate 'senderImagePath contains[cd] "TimeMachine"' --info --debug
In the log, I found this fatal error message:
2023-08-20 00:50:24.361399+0200 localhost backupd: (TimeMachine)
[com.apple.TimeMachine:CopyProgress] Fatal failure to copy '/Volumes/com.apple.TimeMach
ine.localsnapshots/Backups.backupdb/WonderMac/2023-08-20-004329/Macintosh HD - Data/App
lications/Microsoft PowerPoint.app/Contents/Resources/ar.lproj/strman_e644.strings' to
'/Volumes/Backups von WonderMac 1/2023-08-20-004332.inprogress/Macintosh HD - Data/Appl
error: -43, srcErr: NO
(reformatted for better legibility)
So Time Machine complained that it could not copy a file within the Microsoft PowerPoint app. I had in fact run the Microsoft Updater since the last Time Machine backup, so there might have been changes, but:
% ls -al "/Applications/Microsoft PowerPoint.app/Contents/Resources/ar.lproj/strman_e644.strings"
-rw-rw-r-- 1 root wheel 217 12 Sep 2021 /Applications/Microsoft PowerPoint.app/Contents/Resources/ar.lproj/strman_e644.strings
No idea why Time Machine complained about it. The file existed and was world readable.
Solution / work-around
Add an exclusion from Time Machine for the problem file (System Preferences > Time Machine > Options > +) as suggested here for a similar issue, and re-ran the backup. It should now be able to finish successfully.
The weird part is that, after the successful backup, you can remove the exclusion, run another backup, and that one should then also finish without issues.
The solution worked for me, but I'm not sure what to conclude. I have no idea why Time Machine had an issue with this specific file. And why it doesn't when afterwards the exclusion is removed again.