I read the thread you mentioned before I posted this thread. I couldn't find an answer there. The thread reads:
There are only a few possible causes for this error:
You tried to connect to the wrong port. This is a common problem.
You tried to connect to the wrong destination host address.
The server application isn't running on the destination host.
The server application isn't listening on the right port.
I can't see how any of these can be the case, because I have tried several servers on Dalnet, Quakenet and Ircnet, all with the same resolution.