The MediaViewer currently does not work in the German Wikipedia. Instead, when I click on an image, the file description page is opened.
I just saw that the MediaViewer option in my settings was disabled, though I am sure that I did not disable it manually. Also I am not the only user who had this problem. It seems that this option was disabled by enabling a Beta feature, but I cannot reliably reproduce the problem.
I’ll close this task until I can reproduce the problem.