Originally Posted by MakeMeGrrrrowl

.....

Basically, Wordpress was updated and they killed off some old jQuery code that was deprecated. Some of the plugins you use rely on this old code so they were breaking the page. As a temporary fix, I installed a jQuery migration plugin to help fix the issue - and it seems to have (though it does give a nasty list of deprecation notices).

.....

That fixed my issues.

Originally Posted by Colmike7

Just random, but sometimes people or a theme will have something like this in functions, which can make things in admin not work:
wp_deregister_script('jquery');

Then this can fix it if it's the cause:
if ( !is_admin() ) wp_deregister_script('jquery');

Or use this if you want to use the newest jquery:
if (!is_admin()) {
wp_deregister_script('jquery');
wp_register_script('jquery', ("https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"), false);
wp_enqueue_script('jquery');
}


But, so many other things can cause this, and we don't have access to check for sure, so it's just a guess.