jQuery is not defined [closed]

As you have pointed out; the problem fixes itself if you change to another theme. That says to me that your current theme is stopping jquery from loading, hence why you are getting jQuery is not defined.

Look in your theme files and make sure you don’t have something like this: wp_deregister_script('jquery');. You should never de-register it from the admin. If you want to de-register jquery on the front end but not the admin screen then use something like this

if(!is_admin()) {
    wp_deregister_script('jquery'); 
}