So, if openHABian supports Raspi 1 then I must say: it doesn’t work!
My impression was, that the openhabian installation script run into a timeout when starting the service at the end of the script due to the lame RPi 1. I will not retry, since I now have a better suited RPi 3B which works well. I don’t recommend using a RPi 1 for openHABian. It took ages (more than an hour) to install just to fail at the end.