Hello Cubender,
thanks for your reply, but as I tried to explain in my first post, everything was correctly setup.
Finally I was able to fix the problem.
I found in Users/…/appdata/roaming/Steinberg two folders Halion 6 Standalone_64 and Halion 6_64 that contained some preferences.
I renamed these folders (just to have them available in case…) and copied the folders Halion Sonic Standalone_64 and Halion Sonic_64 to the correspondig Halion 6 folders.
Now everything is ok again.
Best regards and thanks agian.