Every once in a while, Cyrus IMAP will die due to a failure in the mailboxes.db file database data. This problem is typically cause by a read/write failure to the mailboxes.db file, which can occur from situations like a power outage during a write to the file.
This can lead to corrupt database indexing data in the file.
When setting up a new Leopard machine, I couldn't get any of my sub-mailboxes to show up under the mailboxes as they were on my other machines.