Thnx Paul!
The UART is now working. What i did:
1. Added this to config.txt:
enable_uart=1
2. Changed the console entry in cmdline.txt to (not sure if this is actual required):
serial0,115200
No other changes were made. I tested the UART communications with a USB to TTL adapter (FTD1232).
I can connect by SSH through Wifi and from there send commands to the UART serial port:
echo "Hello!" > /dev/ttyS0
Which shows up in the serial port console.
Unfortunately, it is not possible to send commands from the serial port console to the raspberry.
Does anybody know if / how this can be enabled?