Fixing (some) access errors in Veeam

I just spent a couple of hours troubleshooting a stupid problem where I got access errors when trying to backup a VM from a newly-installed Veeam server. Searching forums for answers I got red herrings all over the place, from opening up the Windows Firewall for RPC traffic, to removing Veeam VSS files from various folders and shares, to purging keys in the Registry.

It turned out none of that was the cause of the problem, but instead I had re-discovered an issue I’ve seen before: For some reason, Veeam sometimes won’t work properly with UPN logons (username@domain) but instead requires Down-Level logon names (DOMAIN\username). Changing that fixed the problem.