I've had the very same error occur in the last couple of weeks - I spent some time on the Microsoft Website (Uncle Bill's Domain) and it seems that it relates to the ability of the server to take the database into single-user mode (which it allegedly needs to perform some maintenance tasks (integrity checks in my case)). On my SQL box, it seems that a BackupExec job runs under the NT System Account on the SQL's local OS - this is preventing single-user mode. We are currently looking to kill this BackupExec job for a couple of hours on a Sunday so that the integrity checks can be run. I hope this helps,

Diarmuid.