As already discussed via email: caching was interrupted when I lost the WiFi connection. I later resumed it manually and it seemingly worked (the download queue emptied).
But for some tracks, I encountered a playback problem. I tried to remove that album from offline cache, which did not work immediately (it took some time). Later the album was shown as not cached, and I added it to the cache again. Only one out of three three tracks plays properly now.
At some point I also triggered the offline cache job, which led to several files from the last sync being cached again. But there are still some that do not work.
I guess I will remove all of them from the cache and add them again.