if anyone wants to know: the ctrl+c+d-thing also doesn't work...
maybe it's a sign for me that I shouldn't buy so much hardware (new pc, new laptop, 5.1 soundsys, 2 razer mice, sennheiser headphones, etc pp)...
Not at all. There are some other combinations you should try, and a couple tests, before you give up. It is possible that the firmware is damaged, and if so, you may want to try the following:
Remove the AC adapter, remove the battery, press the [reset] button,
without installing the battery - set the battery door cover latch to closed, press [Q] and [T] while installing the AC plug. The amber and green LED should come on, then blink. This is the factory NAND flash mode.
If this works, you will need to go to the trisoft.de site, get the NAND flash image, and copy it to a blank FAT16 formatted 256M SD or CF card. From there you'll be able to try the [D] [M] (diagnostic menu) page, and even be able to get back to the [OK] [power] factory install.
Good luck!
-Martin