It's telling you it stopped because it can't back up
/Users/…/Library/Mail/V10/…/Attachments/…/01_20251221171717017.jpg
It's missing, corrupted, been moved something like that.
You could try running it again, you could force mail to rebuild the mailboxes, you could exclude mail from time machine back up, you could search for the jpg and find out it it's there or what's happened to it....