I had controlled an external synth from Octatrack MIDI sequencer, and the slave was on channel 4. Where the clap on the RYTM is. I did set the RYTM to not receive note messages in midi port config. Also all CC /NRPN are disabled.
I tryed to use the aftertouch controller from a Octatrack midi track, and found that the clap on the RYTM gets more pronounced i.e. he listens still to standard midi messages.