Updating Fail

I manage two websites via Firefox on a Windows 10 PC, and both have been updated to WordPress 5.2.2, using the Customizr theme. This version of WordPress incorporates the Gutenberg editor, which works fine on one of the websites. However, on the other website it gives an ‘Updating Fail’ error whenever I try saving a page.

The same problem exists on all pages. I have tried numerous fixes, as suggested on various forums including:

I have installed the Classic Editor plug-in. This fixes the problem but doesn’t give me all to good advantages of the Gutenberg editor

I have de-activated all plug-ins. This made no difference

I have tried various setting for Permalinks, to no avail

I have configured Firefox to show a Browser Console to monitor Javascript. When I try to save a page the console doesn’t show any errors, but it does log two warnings:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://www.wokinghamlions.org.uk/wp-json/wp/v2/pages/1401?_locale=user. (Reason: missing token 'x-http-method-override' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel).

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://www.wokinghamlions.org.uk/wp-json/wp/v2/pages/1401?_locale=user. (Reason: CORS request did not succeed).

I tried to locate the relevant file via FileZilla, but searches don’t find the wp-json folder. Could this be the problem?

It is obviously a problem with my website, since the other website I manage, http://www.xccommc.com works fine. How can I find the cause of the problem?

The page I need help with: [log in to see the link]