“Timeout expired” errors may be result of a wide range of issues such as network delays, application hangs, database locks. I have recently worked on an issue that was the result of ADO.NET database pool exhaustion.
The entire error message we captured in the DebugDiag dump file:
Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.