Information Monitored Server - SQL Server Collection Execution Failure 01/23/2011 1:23:45 PM Collection 'Session Trace' failed : Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.Severity Alarm Raised Message Snoozed By Snoozed Until
This warning just let you know that a collection failed. When it takes more than 5 min for a collection to get data, the Diagnostic Server deliberately fails it in order to avoid performance problems; it is a part of the product design.
Usually such problems occur when monitored servers are heavily loaded. If this warning is rarely reported , you can just ignored it.