That error message does not seem like it would be thrown on that line of code. You are right it is probably a firewall blocking you. Just because a user says they don't have a firewall does not mean they aren't using one. I know a lot of end users who have no idea that the windows firewall is enabled.

You should never recommend to people to disable their firewalls, you should have them add an exception for your program if they want to use it.

If you are truly getting an error connecting to a website, there might not be much you can do. It could be a router or firewall blocking the outbound connection. Especially in business environments most connections on nonstandard ports will be blocked.

The error screen that you posted seems as if the program is trying to connect to the local computer on port 81. A firewall would definitely put a stop to that kind of connection.