First, thanks for your answers Axel!
If you want to have a more stable environment, you have to boot into safemode, that doesn't start the app_server and runs more or less well.Hmm, if I try to start in savemode or any 32bit vesa video mode, the system resets the laptop after a very short read from HD (I see the bootlogo for about a half of a second).
Currently, you need to change it in the source. In src/servers/app/
Desktop.cpp in method Desktop::AddDriver() the resolution is set.
Just change that to something you like better.
Thanks, worked fine.
Greetings, Chris