Solved!
I had the same problem, and realised that it was due to the unfiltered_html permission being available to admins and editors, but not contributors.
The permission is https://codex.ww.wp.xz.cn/Roles_and_Capabilities#unfiltered_html
I have close control over my contributors, so I actually gave them that single permission and it fixed the issue. I’m sure there are many many ways of enabling that permission, but I did it with the “WPFront User Role Editor” plugin which I already had installed.
I expect that would actually disable your caching if you just renamed object-cache.php and did nothing else.
I did not try deleting/reinstalling (which would probably work since it would re-install back the appropriate caching files), but I did do the rename trick, that allowed me access to the admin panel, then disabled my caching (SG SuperCacher – Memcached option) then re-renamed the caching file back to object-cache.php, then re-enabled my caching. And everything is working fine again.
