Unable to adjust maintenance page with Upfront builder

Hi all,

I have installed a Wordpress Multisite and I have installed the Issue magazine theme. When I adjust the maintenance page for the main website everything works fine but once I want to edit the maintenance page of the added websites it opens but the upfront builder isn't loading. When I check the url it does end with "?editmode=true" so it should be able to edit right?

There is something to mention and that is that I have tried to copy the maintenance page of my main website to all the other sub websites with the plugin "Multisite Content Copier". I didn't wanted to remake the page for every website. First I deleted the maintenance page of each sub websites and then copied the page of the main website using the plugin. The page was succesfully copied but when opening the page it did open an empty page. I have deleted the copied pages again and restored the original maintenance pages. So when I want to edit these page it opens but the upfront editor is not showing. Any idea how to fix this?