Is this specific to one instance of ArcGIS Pro perhaps? Or possibly an issue with the cloned Python environment? Like Anthony mentioned, ArcGIS Pro 2.4 with the 2.4 untools works fine for me.
I would try removing the cloned Python environment, cloning a new environment from Default, installing the untools again, and seeing what you get. And just to make sure, you have either a Standard or Advanced license of Pro correct? The tool will not work with a Basic license.
I did everything you mentioned. Also, I removed the software and deleted anything related to Pro including registry entries. It still doesn't work for me. There's probably something wrong with my computer. I try it on my laptop and it works fine there.