Symptom:I can access the local SQL Express server instance without a problem using the SMSS, but from my application the SQL connection times out almost immediately.
Cause:In my situation, it was a connection string issue. In production I use a mirrored connection. So just out of a laziness, I just change two of the hosts in the connection string to be the same.
Data Source=localhost;Failover Partner=localhost;Initial....
Apparently the driver does not like this type of conneciton string and give you an immediate timeout error.