Do you happen to use SqlDependency to communicate with the Service Broker? I ask because I have been fighting to resolve the same error in my application. This happens when the SqlDependency is stopped and there are unfired dependencies still registered with the database. So far, it has sounded like this is a known issue with SqlDependency. I am currently looking for a way to remove these, and so far I have come up with running the following before starting a new SqlDependency:

KILL QUERY NOTIFICATION SUBSCRIPTION ALL;

It would be more effective once I work through how to determine which ones are orphaned and only kill those. Note you will need sysadmin privileges to execute the above command.