Thanks musocity! Can you provide exact steps for me to follow to reproduce this crash?

I have updated the message that you receive in the plugin to account for compound time signatures in Reaper so you should no longer see it if you set your Reaper project to 12/8 + 84bpm and plugin to 4/4 + 56bpm. Since compound time signatures are different in notation but are functionally the same as simple time signatures, I would recommend that you simply choose a time signature and tempo that match the plugin. This will reduce the confusion around adjusting the plugin tempo to match a compound time signature in the DAW, but will sound the exact same as what you are currently doing.