As soon as I enable the Customise Detail Category plugin, the Appearance>Customizer won’t load – the You are Customising [website] element appears top left, but the page loading bar on Safari stays at about 45%.
It also does this with the other Category plugin as well.
It seems you have a large amount of post category on your website. Please try to use JNews Extend Category option instead. You can find the override option on category edit page. Let me know if you have further question.