200: parsererror

I having a problem in my website when I update the plugin, so I have to keep it overdated. So I’m runing tests in a staging environments. I updated all the PDF Invoice plugins (Basic, Professional, and Premium Templates), deactivated all the plugins and I’m using a basic theme, so it’s a clean environtment, yet the plugin show me a “200: parsererror” in the preview (no error on the console) and the customizer looks broken and the console shows an error 403 on the admin.ajax.php. Any idea what could be the problem?

The page I need help with: [log in to see the link]