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.