You might want to have a word with your hosts about this. On some hosting environments, aggressive settings for the "mod_pagespeed" addon can break the javascript code and cause scripts to not be able to function. This can cause many problems, such as widgets not working, menus not being draggable, the customizer not working, or the media screens not being available.
The fix is suggested is really a bit of a hack. To fix the issue correctly you will need to disable mod_pagespeed, or adjust its settings to not take effect in the wp-admin directories. Ask your hosting service for help on how to do this.