Just a quick information.
Currently there could be an issue with Veeam Backup for Microsoft 365 and Sharepoint. Some sites will be backed up with the following warning:
Processing site XYZ finished with warning: Failed to backup list: XYZ. Unknown Error
I have seen this with 2 different customers and Veeam support is aware of this issue and actively working with Microsoft on it. At the moment I don’t have any further information, but I’ll keep updating this post.
I just found out that there's already a hotfix available: https://www.veeam.com/kb4326
I haven't tested it so far, but will report back next week. Also here's the corresponding R&D forums post: https://forums.veeam.com/veeam-backup-for-microsoft-365-f47/warning-on-my-sharepoint-site-t81314.html
It looks like the issue itself is gone since Saturday; perhaps Microsoft did change something on their side. I did implement the hotfix anyways and experience any problems.