I noticed that the SQL server was rejecting logins from the DOMAIN\UM_SERVER$ account -- the AD machine account for my update manager server. While beating my head against a wall, I decided to create a SQL login for that machine account on the UM database and granted it DBO.
Everything seemed to work after that: I'm using Windows Authentication, the UM service is running as LOCALSYSTEM on the UM server, and the SQL server is on a remote box. The plugin will now load in vCenter and it isn't complaining.
Hope that helps -- bear in mind that I'm in no way a SQL person and would love to know if I just created some kind of nasty security hole ![]()
Edit: It was very helpful to me to look at the vmware-vum-server-log4cpp.log file located here
C:\Documents and Settings\All Users\Application Data\VMware\VMware Update Manager\Logs
on my UM server.