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Did you ever find a solution to this problem? I'm getting it now on M2.3.5pl2.
Thanks,
Fran.
EDIT: Solution is here: https://community.magento.com/t5/Magento-2-x-Admin-Configuration/Admin-Changing-Theme-Content-Design...
Details:
I was able to find the solution and it was mod_security. I edited my modsecurity.conf file
From: SecRuleEngine On
To: SecRuleEngine Off
You can also disable mod_security on your server and it works. On a CentOS server running CWP7pro Admin, it's in "Security / Mod Security".