Time Machine backup error on Mac

You can try deleting the offending file. Finder > Go menu > Go to Folder and paste in the path:

~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Files/SO/5/Attachments/

Then locate and delete the file.

In this case, the problematic file happens to be an Outlook email attachment.

First thing to try is restart your Mac then start a TM backup manually. That typically resolves the issue, but if not, please post back.