Can't edit slideshow with more than 3 slides

Not exactly. Site #1 has been my test and development site since 2012. I created Site #2 a few days ago from a raw Joomla! 3.18 install, added a few extensions which I need, JCE, modules anywhere, ZH Google Maps, AS Slider, AS Menu, vvisitcounter, Maxi Breadcrumbs CK ... and Slideshow CK, of course. Then I exported a few tables from Site #1 into Site #2 : users, categories, articles ... For the sake of common sense, I wanted to keep Site #2 as "pure" as possible, to compare both sites and determine what causes the issue as accurately as I can.

At the moment, the situation is as follows :
• Site #1, my development site for years, is complete, has all 900-plus menu items, and has the "3-slide issue".
• Site #2, the pure Joomla! 3.18 install, has all the articles and categories of site #1, only a subset of modules (including 1 test slideshow), a subset of menu items (6, so far) and works absolulety fine.

Forgot to mention above ...

I also have both menu items tables on site #2, the one from site 1 (which helps show the 3-slide issue) and the native one (which works like a charm) ... of course with different prefixes, to make sure they don't conflict each other. When table #1 is used, I can see the 3-slide issue. When table #2 is used, everything works OK. So there is "something" with that table, which I have not clearly identified so far. The only difference is the params field : mediumtext on Table #1, text on Table #2. So I tried to change params from mediumtext to text on table 1 and I still have the issue. I also tried the obvious : restart the server, restart SQL, clear browser cache .. to no avail. Situation unchanged, 3-slide issue on #1, works OK on #2.

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 ...