Thanks for the assistance.
-Tried the permalinks solution and did not work.
-Disabled all my plugins (All In One SEO Pack, Classic Editor, Contact Form 7, WordPress importer).
-WP still does not save/update a page.
-The Classic Editor does not work for me. (Cannot click on the tabs in the classic editor. The classic visual editor shows no content, even though the content is available on the actual page).
-I have not put anything into my .htaccess file to block wp-json requests (it’s just the basic WP htaccess plus a few 307 redirects).
-I have no security plugins.
-I use Cloudflare.
So there must be something blocking it. Not sure if this is a Cloudflare issue, but I can’t see why that should cause a problem – it is used by so many.