Small update: Even without usb network on, it managed to crash really good. It said - an error occured in your application, kindle will restart in 5 sec. I made a screenshot of that, and it flashed, so hopefully that evidence is there later. Funny enough after the restart it appears to work now. Now testing what I need to do to make it crash again.
Might be completely unrelated to USB Network.
Already tried other active content, used CM, JBPatch, opened and read some in a book. Enabled USB Network again. Nada, that game still plays fine.
Yeah, I want it to work, but I would like to know what it, the Kindle or me is doing wrong.