Turning off CKEditor for Anonymous users

Working with Backdrop 1.29.0 on PHP 8.2.2. The Security Review comes up with "Untrusted users are allowed to input dangerous HTML tags."

It is recommended you remove the following tags from roles accessible by untrusted users.

• img

and says to review text formats.

On trying to configure the Basic settings (q=admin/config/content/formats/filtered_html) and unchecking Anonymous (or changing anything else) and then clicking Save Configuration, I get a 403 error.

<strong>Forbidden</strong>

You don't have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

I'm logged in as admin at the time and the same happens with trying to change 

q=admin/config/content/formats/plain_text

and

q=admin/config/content/formats/full_html

All other sections of the admin seem to work fine, so wondering if there may be a setting issue or maybe a bug.

Any ideas? Thanks!