Got another problem:
I connected the Metaboard to my PC and after pressing the reset-button, I was able to install it as a USBasp-device and upload a sketch. Works fine.
Than I disconnected the USB-plug and wrote another sketch.
After reconnecting the USB, it's only a "unknown device" - pressing the reset switch don't help.
ArduinoIDE says: avrdude: error: could not find USB device "USBasp" with vid=0x16c0 pid=0x5dc .
It's like the bootloader isn't enabled anymore!?
After re-burning the bootloader, I can upload exactly one sketch that runs until the Atmega is connected to power, but after uploading the sketch, Arduino couldn't find the device anymore...

Have you any clue for me, Osgeld?