Netvault with Microsoft 365 plugin failing to backup mailbox

We've just had the Microsoft 365 plugin setup and configured on our Netvault server and everything seems to be working ok apart from a single mailbox is failing to backup, it comes up with an error in the job logs.  We can't find anything unusual about the way this mailbox is setup, it is a user mailbox, and the job logs don't provide any help other than the backup failed for this one mailbox.

Has anyone seen this before?