Okay, so after some testing, it boiled down to the python Interpreter being a (in?) Python venv.

I have now installed the same version of python interpreter and libraries system-wide and the Program works. (until it doesn't, which is a thing for another round of research)

--> Therefore, do not expect libraries like pywin32, which go into operating system stuff to fully work inside a venv.

Thanks to the few who tried to help!