Can't edit slideshow with more than 3 slides

very strange issue
if I understand correctly, you are able to reproduce the issue on #2
so you can put this website online with the issue on a temporary server and give me admin+ftp access ?
I need to investigate in deep in the code

does your other modules save without problem ? even modules with big content, example mod_custom with a lot of text

Interesting ... I followed your recommendation #2. I was able to create a custom module with 3,890 words, no pics. When I tried to save a 5,800+ words custom module, I got the following message :

Erreur
L'enregistrement a échoué avec l'erreur suivante : Le contenu dépasse les limites autorisées.

So far I have not looked for a size limit in custom modules.

Fun fact : I tried to save the exact same piece of text; 5,800+ words, in an article, and it saved without a problem.

Re: your suggestion #1, I'm trying to figure out how to build a temporary website, server, whatever. My hoster won't let me do that for free, that's sure !

The test I did with custom modules is not significant. It yields the same result on both test sites, it saves the 3,800 word text, but not the 5,800 one.

Breaking news !!!

On site #2, the one that's supposed to work, I was finally able to reproduce the problem, by simply adding menu items. At about 620 menu items, I got the Slideshow CK editor issue again ! Apart from both tables being rather large (modules : 3.6 Mio, menu : 1.8 Mio), I fail to see the relationship, if any, between those 2. Oh, and yes, they both use JSON. From what I've seen, of the 10 largest tables in the DB, only those 2, menu and modules, use JSON, plus a 3rd one, ucm_content, which looks empty. Could there possibly be an issue with the aggregate size of JSON data structures ? Just me thinking aloud ...

interesting
I don't know for now the relationship between Slideshow CK and the size of the menu items table
the only idea that I have is that you have all your menu items in the assignment options, then also all the options from the slideshow and this makes the request too heavy
This could be related to a POST size setting on the server or something like that

I found this in my php.ini file. Is that the POST size you were referring to ?

post_max_size=128M

I'll try to play with it and check if it changes anything.