This error came up to me after installing an Exchange 2010 CAS/HUB server and a Mailbox server.
The server doesn’t show any databases; by default the server itself creates an “example” database like Mailbox Database 4385492.
But it didn’t show up!
After running ExBPA I found out that the ‘homeMDB’ value for System Attendant service was missing. This caused the error.
By following this article on technet the error went away:
In addition to what I wrote earlier in the post, I’ll update it here with a picture of the path in adsiedit because of the comments to the post.
Hope it will help to find it.