icloud backup failed - files unavailabe

I just upgraded to an iphone15 and my iCloud auto-backups are consistently failing, saying some “files were unavailable.”

If I manually force a backup, it does work (it didn’t the first time, but has subsequent times). That said, automatic backup continues to fail and deliver that error message.

I have read other threads, but am still curious if there is a more recent solution to this. Thank you!