netgen.gui/ngsolve module error on mac

after installing NGSolve on macOS Catilina I ran into problems with importing modules like ngsolve and netgen.gui (e.g. ModuleNotFoundError: No module named ‘netgen.gui’). I followed the instructions given (setting environment variables and pip install jupyter) and also the NGSolve application runs fine.
Any idea on what could be the problem?