Ok, finally we solved the problem.
Thank you Anamaria for your support.

The solution:
The problem was a mod_security that assign a wrong value to a specific parameter (How you can see here: https://forum.tagdiv.com/tagdiv-composer-not-working/ point 5B).

So I ask to the server support help me, we try before deactivating the mod_security and ta-da! The Error500 disapeared and I can edit the Home Page wit tagDIV editor.
Later the support of my server config the mod_security with default value, so actually the website work fine and the mod_security continue active.

Hello Anamaria,

the guide of general Error500 not solve the problem, the problem is localizated in tagDiv Cloud Library plugin (if deativate it, the problem is solved but obviusly the theme need this plugin).
The requirements are all checked.

I checked in the forum and is completly full of post with user with the same or similar problem with this plugin: https://forum.tagdiv.com/search/tagDiv+Cloud+Library+plugin/

My problem is similar to thisone: https://forum.tagdiv.com/topic/error-500-when-activating-the-tagdiv-cloud-library-plugin/
But in the post they don’t write how solved it.

Please help me and don’t reply with generic solution.

The error from TagDiv Editor is: Error at zones: 6(models) : 0(dom). The model does not match the content Current state has involved an issue. You could try an UNDO operation (CTRL+Z / CTRL+SHIFT+Z)