Okay, I found the issue:
Xibo is using StashPHP with Memcached driver for caching inside Docker.
Any Cache->save() failure seems to be ignored. I always see the Xibo debug message saveToCache: cached /widget/dataset/af0bbf628e7bce0898a5e821871073a9 for 300 seconds. But the truth is another: Memcached reports SERVER_ERROR object too large for cache.
Xibo is also just checking, if the $this->cache->set($object); returned an error but not if the $this->pool->save($this->cache) returned an error. The latter could pointed me eralier to the right direction maybe…
However: The solution here are slight modifications of the memcached service container:
The default object size is 1m. But my object is 1.61m.
So I changed the default memcahced cmdline from command: memcached -m 15 to command: memcached -m 32 -I 10m.
-m is the memory limit, I dont know if some more would be a problem. -I is the item size limit.
Now its working!
I create a PR for the Check, if the ->save command was successful. I DONT create a PR for the memcached cmdline. I want a dev to check my new parameters.