hi Spajderix!!
Your problem has to do with Teeworlds' default resolution. Since it is lower than what your monitor can display, it just rejects showing it. This is what you gotta do:
1) Go to a Terminal and type: sudo nvidia-settings
2) From all sections at the left find the one that says "GPU 0 - (your video card model)"
3) Inside that section click where it says "DFP-0 - (your monitor model)"
4) Now find the option called "Flat Panel Scaling". Below that check/enable "Force Full GPU Scaling"
5) Just right below that click on whatever "GPU Scaling Method" you want. If you leave your mouse hovered over each scaling method, it will show you a description of what each one does. I personally like Aspect Ratio Scaled
6) Click Quit
7) Now execute Teeworlds
8) Go to the Settings tab, then select Graphics (to the right)
10) Select 1440x900 24-bit
11) Exit Teeworlds
From now on Teeworlds will execute on the resolution you just selected and your monitor will show anything you throw at it ;P
Hope this works for you man! Good luck!