For me the problem was that my venv was somehow damaged, probably after a VS update, as @user2577923 pointed out. The solution was rather simple:

1. Export the dependencies from the existing venv to a requirements.txt (if not already present)
2. Remove the existing venv
3. Create a new venv, using the right python version and requirements.txt

Now it should work again. I used the python environments extensions to perform these actions, although you could also do this using the command line.