I now re-installed CCU3 and also OpenHAB (version 2.4.0) but nothing changed.
A xml-rpc “device-lister” shows both channels. I would like to provide logs or other information, but I do not find anything suitable for this problem.
The only thing which is unusual, is that only my two HM-LC-Sw2-FM are affected.
If someone points me in the right direction, I will be very thankful.