Dear bartimausi,
That was exactly the hint I was looking for. The other add-ons (ZDF Mediathek, ZDF Mediathek 2016, Unithek) don't have an option to set the desired quality anymore. However, I was lucky with "Mediathek" and the new add-on that you suggested (Mediathek direkt). They work like a charm in "high quality", but crash Kodi in "HD quality".
--> I wonder if we should file a bug on this? If yes, is this something for LibreElec or for Kodi directly?
One more thing:
How do you use these add-ons? Do you find your shows, movies ... the traditional way (tv program...)? What I really like about the ones that don't work anymore is that they offer a recommender (most viewed -- Meistgesehen) that shows you what others are watching. The two add-ons that remain don't offer this. Contrary, they offer access to all the stations in parallel and a unified textual search, which makes it super hard to discover something interesting for me. -- Do you have some "guide" that helps you in finding what you like to watch. (For quite a while I'm hoping that Kodi will make these libraries/repositories (Mediathek) first class citizens like the old sequential TV.)
Thank you so much and
best wishes