... not anymore. I did a full uninstall (Studio and GPO stand alone) and re-installed it again, leaving the already present multis and 9. Notation (library) at their place (where they were in the original setup).
Now I can input notes (playback) via my Midi keyboard. Don't know what happened with some crucial files. I spent the whole afternoon looking for the error and didn't find it, until I decided to re-install everything.
So.... guys, continue having a cold beer, or a refreshin swim, good old Raymond (62 years old...) did it again :-)