[Chivato]

Changed my config.txt and plugged in my USB Wifi stick and rebooted the Pi. Now I get the colored square which indicates power issues.

Going into LibreELEC -> System gives all options ;-). Selected my network from USB Wifi and it automaticly reboots. After reboot I was able to select my network over USB Wifi but the colored square keeps showing up.

So many thanks for this great idea Aux_ !!!! But to be sure I will also purchase another poweradapter.