Yep, it is the correct device. If it wasn't, I wouldn't even see garbage.
ttyAMA0 ist the device on Raspi.
But now I might have found something. I googled "prolific serial to usb garbage" and found a bunch of articles talking about poor hardware and even poorer drivers. Let's see if I have something different in the workshop
I just remembered the usb2ftdi cable which I used to program mini arduinos. fetched it from the workshop and without any further doing I get a login screen.
Thank you very much for your patience with an old man. Problem closed