I've not found much of anything on this out on the internet, so I'm hoping that someone has an idea what could be causing our problem.
Our V7 environment was working through it's normally scheduled jobs like usual, about 2 hours after it started, our VMware SSO server went unresponsive, and the remaining Veeam jobs failed. The tell-tale signs there was an issue with the SSO was that all of the failed jobs came back with an error in the report that said something along the lines of invalid username or password, and no one could log into vSphere. Someone signed into the SSO server, restarted the SSO service, and everything was good again. Unfortunately, since this is V7, there's no Snapshot Hunter, so there were a handful of "VEEAM TEMPORARY BACKUP SNAPSHOT" still hanging around, including one on a 13TB VM. The following night, Veeam kicked off, one of our hosts started generating NMI errors, and then PSOD about 20 minutes later.
I'm certain that upgrading from V7 to V8, V9 or V9.5 would solve the lingering snapshot issue, but I have no idea what would have locked up the SSO server. This seems to happen every 4 or 5 months.