Yesterday, I followed the Joomla Docs instructions by copying a live site and installing it on a development platform (a subdomain on another domain), upgraded to J3.10 then to J4 Beta 5. All non-J4 compatible plugins, extensions (e.g. Regular Labs) have either been disabled or uninstalled.
The site is rendering correctly and I'm able to edit template files in the admin panel.
However, when I click on 'Menus -> Main Menu' I just get this:

None of my template files contain 'JResponse' and clicking the 'Rebuild' button does nothing.
If I start from scratch with a new installation of J4 Beta 5 I don't get this problem. Any ideas? Thanks.