Upfront interface not working

I am no longer able to edit my Upfront theme. When I go into the Upfront editor from wp-admin I just get the website itself. This goes for editing the current theme as well as using Builder or changing to another theme.

I have tried disabling all plugins, clearing the upfront cache and switching themes.

I have also followed the discussion here, and was advised to create a new ticket after posting a comment: https://premium.wpmudev.org/forums/topic/upfront-interface-breaks-after-adding-second-image#post-1178258

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