I downloaded your site using All in one migration plugin, I uninstalled it right after testing and the issue is still present as well. Though, with clean installation, it works just fine.
So the cause of the issue is internal to your site which I couldn't find as of this moment, plus, your site size is 4gb, it's a bit big.
Perhaps you can clean your sites first removing unwanted files and data? It's to slow as well so this could be related to performance, and since it's big, increasing memory limit may not work.
How about installing a clean Wordpress in a second staging, then export and import your pages as templates? Then test the builder again.