Posting from a Client Throws an Error Message (in the client)

Posting a notification from a client works, but returns error to the client. E.g., When I post from Twhirl, Twhirl returns a message that the posting failed, but the message is posted, and Twhirl picks it up on the next refresh. In Adium, it returns an "Unknown message error" message, but the post goes through.

Any idea what message is (or isn't) being sent back to the client to cause this?

Thanks.