https://wordpress.org/plugins/user-access-manager/
When this plugin is installed activated will break the functioning of the Template Customizer, it will briefly load and then disappear. The only known fix at this time is to deactivate this plugin. This might be a good idea because at the time of writing this FAQ it has been over 2 years since it has been updated and there are many reports of it breaking other plugins and not working correctly in newer versions of WordPress.
NOTE: This FAQ was written based on Root Relative URLs 2.3