The latest upate was sent from another help desk person, with the excuse "since the other person is not available I am replying on his behalf" - typical of help desk services where there's no ownership of cases and people simply stop answering when they don't know the answer.

His suggestion is to "reboot the server and try the backup again and try starting the backup three times. If the issue still persists then contact them again".

Very bad - not only counting this is a HyperV host with a few Virtual Machines running on it, I have the impression "User Account in use" is something on their side.

Oh well. Got an email from a Sr Marketing Manager after complaining on Twitter. Forwarding this information to them.