I had a similar issue. We upgraded our Exchange cluster form Windows 2000 Advanced Server SP4 (I think?) / Exchange 2003 SP1 to Windows Server 2003 Enterprise Edition SP1 / Exchange 2003 SP1. Before the upgrade, I could backup the Exchange DB but after upgrading the OS, I could not. When I would go into the backup job, I could not select the DB, it did not show up. Once I enabled restrict anonymous support, I was able to backup the DBs.
-Colin