With a workaround, probcurv now works in Ableton!

The setup:

Channel 1 should contain probcurv on its effects chain, but not have a midi instrument.

Channel 2 should have the midi instrument you want to use probcurv with.  It should take the MIDI signal from Channel 1 as input, like so:

If you've set it up correctly, the MIDI input to your instrument should be processed by probcurv!  This fix coming to you courtesy of JUCE 6, which I have migrated to and thusly upgraded the build for.  Download is updated!  Can't wait to see what you make with it.