Ok, I was able to solve it myself. I didn’t recompile PETSC as suggested, given that I had not built from source in the first place, so I thought the problem might be elsewhere.
The message was complaining about pyvistaqt. I tried to install it, but then the demo was giving the error No Qt bindings could be found.
I installed pyqt5 using pip (it wasn’t available from conda):
python -m pip install pyqt5
I noticed matplotlib wasn’t in the environment, so I installed it (curiously, the first demo run).
After that, the demo worked. Leaving it here in case it helps someone else.