>>>>
>>>>You are right about the Agent.
>>>>If SQL Server Express is installed on named instance (which is so) and not use port 1433, how do I find out what port is used? And how to I find out what is blocking user access to the SQL?
>>>>Thank you.
>>>
>>>Ok, I found out (from Task Manager) the port on which SQL Server is listening. But still no solution to what is blocking it. The customer IT says there is no Firewall. But they do have Trend Micro AV but I don't know how to deal with it.
>>
>>Is the SQL Server Browser service running?
>
>Yes, the SQL Server Browser service was running. And they (IT) sent me email this morning that the problem has been resolved.

Great, I just remembered that one of our clients had a problem with a corrupted installation and the SQL Server Browser did not start, and they spent hours trying to figure out why the client could not connect. So I have a checklist that is displayed in the application, when there is no connection to SQL Server, and they can go through this list before they give us a call:

1) The server address and/or instance name are incorrect
2) The password you entered is incorrect
3) The sa user (or which ever user account specified) is disabled
4) The SQL Server client driver is not installed (the one specified in the connection string)
5) The firewall on the server and/or client blocks port 1433
6) The SQL Server instance does not allow remote connections
7) The SQL Server configuration has TCP/IP disabled
8) The SQL Server Browser service is not running on the server

For sure there can be more reasons, but this is now my top list of occurrences that I see in practice.

By the way it is usually a bad idea to change the default port, unless somebody really forces you to do so.