I just ran into this on a VeeamOne 9.5 server... I added a 6.5 vCenter to my environment because I couldn't upgrade the existing 5.5 appliance (long story) and when I went into Monitor and deleted the old vcenter, it sat there doing nothing for 45+ minutes (the old vcenter was italicized and grayed-out, but it never went away). Then I rebooted and now I get the "Unable to process request because database is disconnected" error when launching VeeamOne Monitor.
The server is also running like crap... really sluggish. Barely any CPU or network traffic, but lots of disk for SQL Server. I think VeeamOne must be deleting every record in the db ONE BY ONE related to the old vcenter, in some terribly inefficient manner.