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Info source: https://pluginus.net/support/topic/description-content-is-not-saved (thanks to alien2108)
If I want to copy media rich Description to another article I get "Warning Saving...", but no green notice "Notice Saved". Of course nothing is saved. This happens in both Graphics or Text view attempt. If I try to copy Description in text mode I even get the "Notice Saved" info, but it is not saved.
Yes, by disabling WAF in Plesk everything worked. This rule id: 33350147 is the reason (you can see that in logs of WAF) and according to google search happens quite often.
So I disabled just this rule and enabled WAF back and everything still works now.
I would suggest you try to at least report an error (if you can) when you get 403 back, since now it was not even clear to the user what happened...no errors were reported.