I'm experiencing this problem and cannot work when midi editor is opened.When closed everything is ok.I'm on latest Reaper x64 (tried other versions its the same).
Windows 10 x64
RME 800
Reaper 5.22 x64 (and any other version)
here is the loopback record of whats going on.it starts when midi editor is opened and then I close it after:
https://stash.reaper.fm/28215/midieditor.mp3
I don't know when this problem started because I don't work much with midi,but the newest thing I've got In my setup is RME 800