Recently we faced an issue of this message filling up the error log :
An exception occurred while enqueueing a message in the target queue. Error: 15517, State: 1. Cannot execute as the database principal because the principal “dbo” does not exist, this type of principal cannot be impersonated, or you do not have permission.
The solution actually has to do with a broken Database Authorization.
Simply change your database authorization back to sa, and your error is solved.