thanks to everyone who has helped me.

It actually turned out that the ffmpeg release I installed as an update did not contain the libmp3lame. To fix this, I had to download an alternative release and after deleting the old installation folder, install it in a new folder and set the system path variable again. Simply putting the new release of ffmpeg in the old installation folder did not work, strangely enough. Ffmpeg then insisted that libmp3lame was not installed...

In short, now everything works as it should.