It is definitely in the correct installation location, in the user data plugins folder that C4D sets up, since I can see another working plugin in the same location show up in both the plugin manager and in the UI.
Again, I am simply copying the plugin folder from the R19 location to the R20 and R21 location. Nothing else is changing. It could be an encoding issue that R19 and its version of python handles, but that R20 & R21 don't. I was looking into that last night before you mentioned it. I'll keep checking that.