[Milhouse]

But this does not explain why it also deleted the correct entries. The movie Avatar is in the library, and when I look at the info screen, it also shows the path smb://syno/filme/Avatar (2009). I can actually also play it from there. But somehow the vclean and P deleted those entries, so the movie is still in the library, but its pictures got deleted.

vclean is just running the Kodi clean library function, what it deletes is down to Kodi.

As for the prune command, I can't say why it would delete an artwork item without knowing what is in (or was in) your library, and what has been removed from the cache.

Many times I've looked at the output of the prune ("P") command and wondered why it would delete a particular artwork from the cache as I'm sure it should be associated with a movie in the library, only to discover that the prune function is correct and the movie is associated with an alternative artwork item.

Dump your movies with "texturecache.py jd movies >/tmp/movies.dat" and see if any of the pruned artwork is in the movies.dat file - I'm pretty sure you won't find them, which is why they were pruned. If I'm wrong, please upload movies.dat somewhere along with a list of the removed artwork.