Filepicker spins forever

When I (the teacher) or a student wants to upload a file using the browse button in the file picker, the circle just spins forever. Using drag and drop to upload a file works fine, but is a bit clumsy for people working with only one screen.

I can reproduce this by going to a course, adding a file, and using the browse button in the file picker. For students, the problem appears for instance while using assignments, where a file should be handed in.