AJAX not processed even when “success”= true

Hello !
I have a quite blocking problem with the latest WordPress with only a minimum number of plugins (WordFence, Shapely theme, Jetpack, SiteOrigin Builder and W3 Total Cache) everything running on the latest Nginx + PHP-FPM 7.2.2 x64 on Arch Linux behind CloudFlare.
In lots of pages (especially administration) that requires realtime loading with AJAX, the request is correctly sent, processed, and received, but not interpreted with the JS.
{"success":true,"data":[{"id":35,"title":"XXXXXXImageNameXXXXXX","filename":"XXXXXXImageNameXXXXXX.jpg","url":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-content\/uploads\/2018\/02\/XXXXXXImageNameXXXXXX.jpg","link":"https:\/\/XXXXXXMyWPAdressXXXXXX\/XXXXXXImageNameXXXXXX\/","alt":"","author":"1","description":"","caption":"","name":"XXXXXXImageNameXXXXXX","status":"inherit","uploadedTo":0,"date":1518628230000,"modified":1518628230000,"menuOrder":0,"mime":"image\/jpeg","type":"image","subtype":"jpeg","icon":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-includes\/images\/media\/default.png","dateFormatted":"14 f\u00e9vrier 2018","nonces":{"update":"66acf7cdb8","delete":"ab3dfdb163","edit":"6919e5f50b"},"editLink":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-admin\/post.php?post=35&action=edit","meta":false,"authorName":"XXXXXXMyWPUsernameXXXXXX","filesizeInBytes":777332,"filesizeHumanReadable":"759 KB","context":"","height":1080,"width":1920,"orientation":"landscape","sizes":{"thumbnail":{"height":150,"width":150,"url":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-content\/uploads\/2018\/02\/XXXXXXImageNameXXXXXX-150x150.jpg","orientation":"landscape"},"medium":{"height":169,"width":300,"url":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-content\/uploads\/2018\/02\/XXXXXXImageNameXXXXXX-300x169.jpg","orientation":"landscape"},"large":{"height":576,"width":1024,"url":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-content\/uploads\/2018\/02\/XXXXXXImageNameXXXXXX-1024x576.jpg","orientation":"landscape"},"full":{"url":"https:\/\/XXXXXXMyWPAdressXXXXXX\/wp-content\/uploads\/2018\/02\/XXXXXXImageNameXXXXXX.jpg","height":1080,"width":1920,"orientation":"landscape"}},"compat":{"item":"","meta":""}}]}
As you can see here, the request is correct and indicate a success.
However, the gallery (in this example) only show the loading circle and does not show the images in grid mode. There isn’t any error in Nginx logs, nor WordPress in full debug mode.
Another side effect is when “Personnalizing” the theme : modifications are not saved, and it only show an error even if the received request indicates a success (and BTW, modifications are really not saved). When installing extensions, it shows again an error message, but the extensioon is installed.

I tried modifying the JS wp-util.js by hand but it did not change anything. Also tried to disable extensions, nothing too.

Is this a WordPress bug ?

Thank you in advance,

Arno

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