Microsoft SQL Express uses dynamic ports. Confirm that the client workstation can telnet this port by using the port number in the ODBC connection.
The port used by SQL Express may be closed on the Firewall.
The error message may be resolved as follows:

Next, go to the ODBC DSN configuration window and create or modify your DSN configuration
For the Server name, use the following [server]\[instance],[port number]
Example:
