Although I haven’t done this course yet and therefore can’t help more directly, I’ve read about this problem before, and I’m pretty sure it has to do with the underlying python files getting mismatched and no longer pointing to the resources they’re supposed to (analogous to the infamous magenta texture for missing files).
Edit: I managed to find the original post. If FedPete’s solution doesn’t work, have a look at the link I put there, as it connects to several other possible solutions: