Mailbox switch fails, with the error "Error 1000 Invalid parameter 'Entry.HomeServer', value 'nul (4255123)

When attempting to switch a mailbox, either automatically through a mail synch collection, or manually by right-clicking and selecting Switch Mailboxes..., switching fails and an error is noted in either the EMWMSA.log or EMWMigration.log such as the following:

2008/03/14 12:53:43 PM CMBMConn::GetMailboxInfo Error 1000 Invalid parameter Entry.HomeServer, value null.
2008/03/14 12:53:43 PM CMBMSwitcher::Switch Warning 4930 Cannot switch a mailbox because the target mailbox has not yet been created. The agent will attempt to switch the mailbox again during the next session.,

Despite the error, the target mailbox is fully created, has been successfully matched by directory synch, and all Exchange attributes are in order.