I'm on B&R 10.0.1.4854 and have been using this for a while without a single issue until yesterday.
I restarted my Hyper-V server yesterday morning and after it rebooted, all my veeam jobs are failing with this error. I've seen some posts in the past referencing to just restart the veeam backup services, however that didn't resolve any issues here. After the job fails, the server shows in backup infrastructure as offline, but performing a rescan shows all volumes appearing online and visible, however starting the job again produces the same results.

I also tried re-installing any missing veeam services by editing the server in the backup infra tab, this didn't seem to do anything as the process completed instantly (successfully) but still results in the same error in the backup job. The actual "upgrade" option for this server is greyed out and unavailable, which I believe indicates that it is fully updated.
I have other jobs going from different hosts that are still working, and they are using a volume from this server's storage as a repository.
All the references to this error I've seen in the forums and elsewhere on the web indicate just restarting the servers and/or the veeam services resolve the issue however this doesn't seem to be the case for me. I'm not sure what else could be causing the issue.