>I did some digging in the backend…and in a sub-sub menu found an error log for the hoster…probably is not caching but rather some security protocol that they have running, no?

and there is your problem.
i.e it’s ‘mod_sec’ (mod security) throwing a false positive , only your host will be able to fix this (having had a bit of a rummage through some sites , and seeing the id being “33340156” (according to your logs), there might be a way to bypass things for this particular mod sec rule (htaccess or some other way)
but that’s really something you have to take up with your host

PS: and before you come back to me telling me that your host says “it’s insecure so we will not do anything about this”
ask them why the *same* script works the next time round when you increase/decrease the quantity after you’ve deleted an item
(not to mention it’s working just fine for thousands of users and i have mod_sec too installed here without any such issues)

happy to have gotten to the root cause of this finally and having it here on the forum is most certainly a good place to perhaps help others
(or even if it’s just for myself at some point to look up again)

most of the time, hosters/host support are quite helpful though with these sort of things
(I don’t want to – yet – randomly accuse people here 🙂 )