Hi there,
finally even “I” 
get the FS20 and CUL fully running.
Thankfully to the very nice preparation of a specific image created for the raspi discussed here:
I´m sure this was one of key cause to exclude anykind of install issues.
Further I have realized that my so far done configs I´ve created always into ONE file opehab.cfg.
This is simply wrong for OH2. In OH2 there is in the runtime an “services” folder which contains all specific .cfg files.
This is the right place to adjust your …/ttyACM0 ect. cul device.
The same thing have to be done as well for the FS20 binding.
Each time this is done into an EXTRA cfg file and not anymore to ONE openhab.cfg like in the past.
Afterwords I would say that´s much more convenient and logical, but took me really a while to find out.
However, if in the Paper UI the binding get installed you can notice for the culintertechno taht a .cfg file is done,
for the FS20 you need to do an own :-/ little bit unlogical that once you get a config file and other time you have to create an own one.
So for all folks out there coming from FHEM as me, simply follow this experience and I hope you can get it faster run than me after long nightmares 