As per Microsoft: "You may see Database in use or Database not found errors on a server on which the Recovery SG Override registry key has been set. For more information about this registry key, see "Recovery SG Override Registry Key" in Chapter 1 of the book Using Exchange Server 2003 Recovery Storage Groups.
Active Directory replication or cache latency can also cause the "Database not found" error. If this is the cause, you may have to wait up to 15 minutes for the local Exchange store cache to be reset. You can clear the local Exchange store cache by stopping and restarting all Exchange services, the IIS Admin Service, and all Windows Management Instrumentation (WMI) services.
Typically, Active Directory replication will also complete in this amount of time, but depending on your Active Directory replication topology and latency, replication may take longer".
Using Exchange Server 2003 Recovery Storage Groups is a book that provides information on how to determine if a recovery storage group is useful in your deployment, how to set up a recovery storage group, and how to troubleshoot common problems. Also check M296841 and M824126 for more details on this error.
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