For the record, I built Martin Zuther's Compressor in Qt yesterday. Dropped the dll into my x86 Plugs Folder and fired up REAPER. When I finished, I closed REAPER and put a video on for my daughter. Couldn't listen it - sound compressed and distorted to hell. Aha, the plugin hasn't let go the Soundcard again! This time I rebooted and disconnected the external soundcard and went with the internal..Later I went back into REAPER, messed about with the Compressor again and closed when finished. No Compression this time, It has let go..
So it's quite possible that my aging Focusrite was the problem. Better write my letter to Santa tonight!!