I am using a windows 7 system and most of the time I get the "could not connect to host…" error, but certain severs I can get into. I am not sure what the cause of this error is, but I am fairly certain that it is not a firewall problem. Restarting the computer does not fix the issue. Any ideas?
The host probably didn't forward his ports/made a firewall exception, so it probably isn't your fault.
Maybe it is me… how do I forward my ports?
Forwarding ports is only important when you're the one hosting the game. It's a setting in your router's web interface. You'd have to forward ports 11112 (TCP) and 11113 (UDP) to your computer.
Until Luchs comes up with a fancy idea to show users with blocked ports a banner upon starting a game, you can use CREMA (Clonk Rage Enhanced Masterserver Analyzer, working title!). It is basically a tool which knocks on all supplied ports to check whether the host did forward them.
That's afaik implemented in OC @banner
It is also possible in Rage: https://github.com/ckanibal/clonk-rage/blob/master/src/network/C4League.cpp#L299
I don't understand this line - it doesn't have anything to do with blocked ports, does it?
I meant that in OC users are actually notified with a banner when their ports seem to be blocked.
I think he means that CR can also display a banner with a message from the Masterserver. If you then implement then banner check in the masterserver, it can send a message to the client on the next reference update/signup.
You can even do it for the current reference - when the client sends the reference to the masterserver, the sockets should already have been established.
Are you sure? I think there's a small window where a game appears in the list, but you can't connect to it yet.
Ich habe CREMA eben abgeschalten, Source (zum selberhosten) gibt’s hier.
Warum? 
I re-hosted it at https://crema.clonkspot.org/
Is it possible to integrate this into the Live games list?
Yes, this is planned - but obviously a lot more work than hosting an existing app.