I see the error "System.Exception was thrown" in a M365 Mail and Archiv Backup-Job.
The logs shows also "Failed to extend mailbox immutability". The Job has a retention of one year and a immutability of 180 days.
Only one Mailbox in this job is affected. All other mailboxes work as expected.
Support is not realy helpfull here! Only one response in 14 days. Requesting updates to the case are also not replied from support.

The Only mention from support was to try the following
But this doesn't help. The newly create job errors out with the same failure,1. Edit the backup job and add the mailbox to the exclusion list
2. Create a new backup job named "Test"
a. Add only the mailbox to the job
b. Point the "Test" job to the same repository as the main job
3. Open PowerShell and run the following:
Get-VBOJob -Name "Test" | Start-VBOJob -RunAsync -Full
4. Once finished successfully:
a. Delete the "Test" job
b. Edit the main job and remove the mailbox from the Exclusion list
Has here anybody any idea about it?
Case #07514282
Greetings Rene