Thanks to EvilDragon for the solution and Sexan (who luckily had the same problem as me ;-))
I wondered why I had crackling without an audio buffer alarm from Reaper and only happened to recognize the connection with open MIDI editor windows.
Is there no way that reaper detects this problem and reports it in the same way as ASIO buffer underruns?
@mozart999uk I als don't understand the term: "editing preview latency" but read, that you should disable "anticipative FX" when recording with very low latency: see
https://forum.cockos.com/showpost.ph...24&postcount=3
But still a mysterium for me how to handle all that settings in a meaningful way.