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Thanks for that!
We are working with multisites.
So there was this one event on dev.kanoon.de (mastersite) which was layering over the events of its multisites music.kanoon.de or games.kanoon.de,...
I deleted the issue causing event (on dev.kanoon.de) and after clearing the cache (the debug facebook link you sent) of the single events in music.kanoon.de the image appears again!
But when I create new events, it won't take the image again. So there must be still a bug in the cache.
Do you know if there's a function or a way to clear all cache so that I don't need to clear the cache of every single event created?