I had the same problem. Here is the explanation: You had too many sent items. When you press the outbox button, the systems sends a query to the database to select all the messages from the IM table sent by yourself. This query is heavy and takes time to be executed from the server. So, if your hosting server has a small timeout setting, it hits this limit and the query cannot be completed. I changed hosting server and now my forum is much faster and heavy queries like this have no problem to be executed anymore!
