Hello again, thanks for your fast response.
We aren't on the same network. We tried all these things we could find on the forums, but we still weren't able to play together. Then my girlfriend had this amazing idea - she just restarted her notebook. But, to my big surprise, it worked. I suppose it was only some sort of glitch or something, I don't know.
Anyways, I'm not sure if this "method" can help someone else with this problem, I'm assuming that rebooting was only needed as a starter to some other fix we tried, but still, who knows...
Once again, thank you for your fast and thorough response, I really appreciate that. Also, I'd like to thumbs up Castle Crashers, it's really an amazing game and a ton of fun.