UPDATE: Please see Update 3, as the hotfix/workaround needs to be undone because of possible side-effects.
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.
Last week Veeam Support did send out a notice regarding the hotfix for this problem. The hotfix could cause possible side-effects and so the changes need to be undone.
After a joint investigation between Microsoft and Veeam®, it has been determined that the workaround may include undesired side-effects. As such, remediating actions are required as outlined below.
The following steps should be applied during the next available maintenance window in your Veeam Backup for Microsoft 365 environment:
1. Revert the product binaries to default:
The easiest way to accomplish this step is to reinstall the product or run an update. You can find the latest product builds here.
2. Perform a full sync of any affected backup addressed by the fix for the aforementioned error. To do so, from PowerShell, run:
Get-VBOJob -Name “Name of the affected job” | Start-VBOJob -RunAsync -Full
It is important to take the above remediating steps as soon as possible. A cumulative patch is being developed by Veeam to change product behavior and avoid undesired consequences. The root cause of the issue has been resolved by Microsoft on the Microsoft 365 side.
So if you did apply the hotfix, make sure that you’re undoing the changes as soon as possible. The problem itself has already been solved by Microsoft, so the errors shouldn’t come back.