As1anSwagger,
What router/firewall do you have? Have you updated the firmware on the device? A firmware update on your router may fix the issue or it may be a hardware issue. Go to the manufacturers site and look up the model. You also may be able to replace this firmware with more reliable firmware such as dd-wrt or tomato.
I have had an issue with a set of firmware bugs on Linksys and D-links that caused the router to crash. In the end it lead me to replace them with devices with much more RAM and flash in them. Generally consumer SOHO (small and home office routers) suck and have very crappy procs, ram, etc. You can test this yourself by doing the following:
Test as follows:
Open up 2 command prompts/consoles and type ping -t
www.google.com (if on windows you need the -t) or ping
www.google.com (Osux or Linux) (leave this window open) in the other ping your default gateway or your router which is usually in the IP space of 192.168.1.x. [You can find out your default gateway (router) by going to a command line/console and typing ipconfig /all or ifconfig -a (Linux/Unix/Bsd/OsuX).]
Start up Xonotic go to your Settings >> Video and then untick Full Screen. Make it so you can see the ping output and then you will see the ping output. If you no longer can get to Google or your default router goes down with either a Destination Host Unreachable or Request Timed Out message - you know that it is your cr@ppy router. =P You can then spend hours on the phone with incompetent support that will do a hard reset on your router because that is all this is listed in the "support" script and then they will hang up on you. @ssH0l3z! Then I would recommend turning your router into a paperweight and buy something that will run dd-wrt or tomato. I'm kiddin'! =X
Seriously, though - let me know how it goes or if you have any questions.
@Vargos
You only need port forwarding if you are setting up a server. Otherwise you don't need to open *ANY* ports on your firewall.