When trying to connect to any server (yes, using the correct ports and I _have_ tried several servers that have worked before and that are located in several different countries), I get the legendary

* Unable to connect (Connection refused)

What could cause this? Feel free to ask stuff, as I don't know what information is useful in order to find a solution. If you could point out an existing suitable thread, I would also be happy.

Thank you for your answers in advance.

You can read info about Connection Refused error in this thread Right now this is pretty common error on DALnet, altho you dont say what network, have you tried some others to see if it happens as well?

I read the thread you mentioned before I posted this thread. I couldn't find an answer there. The thread reads:
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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.
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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.

It also says no connection could be made because the target machine actively refused it, or didnt resolve correctly.

try this:
/server irc.quakenet.eu.org
If you get the same error, try /server 194.159.164.211

Tried those and both didn't work - same error.

alt+o,options, connect, what is the default port ? try 6667 or 7000 for dalnet or give the port number in the /server command. /server servername port#