strange permanent midi stream on IAC port 2 input

[dompaul]

Reaper 5.60 running on macbook pro retina 15" under OS X.11
I currently use IAC ports for recording from multiple keyboard.
I always get a permanent midi stream (always beginning with B6, A#6 notes on, then a lot of various periodic events, including cc messages, aftertouch...), when I launch recording on IAC port 2, no problem on other ports.
This occurs even if there is no other app running on my laptop, even immediately after booting the mac. I tried to initialize IAC driver from audio/midi configuration without any effect.

For now, I have disabled IAC port 2 under midi preferences and added another port via driver settings. All other ports work without any problem.

If someone get an idea...
All the best
dompaul