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.

Thanks

I checked the plugin manager in all 3:
R19: the plugin is in the list.
R20: not in the list
R21: not in the list

If it is not in the list in R20 and R21 does that mean it didn't register?

Thanks

I'll check one more thing before I send it.

I noticed C4D R20 & R21 both use python:
'2.7.14 (default, May 3 2018, 18:05:57) [MSC v.1500 64 bit (AMD64)]'

While R19, where my plugin works, uses python:
'2.7.9 (default, Apr 20 2015, 08:46:13) [MSC v.1700 64 bit (AMD64)]'

This is probably the reason, but why?

I just checked 2 python plugins I developed that both worked fine in R19, but in R20 and R21, one of them doesn't show up. I copied the 2 plugins, without changing anything, from the R19 plugin location to the plugin folders in both R20 and R21. I checked the python console and there were no errors displayed. One of the plugins just goes unnoticed by C4D in both R20 and R21.

Any ideas why?

Thanks,

Chris