can’t update focus keyword

I am testing Rank Math and has setup to use the focus keywords from the title of my post.
Only when I want to edit the focus keyword of a single post the SEO score is updated only when I refresh the page or return back to wp-admin and open the post again the focus keywords won’t be saved.

I found this knowledge base article:
https://rankmath.com/kb/meta-data-not-saving/

And checked for conflicted plugins but that’s no issue.
I don’t use a firewall plugin only iThemes but when I disable that plugin the issue still exists.
There are no javascript errors on the edit post page.
I use cloudflare and have whitelisted the page rule as mentioned in the article.

But the issue still persists.

When I go in the browser to the page: /wp-json/rankmath/v1/updateMeta
I get this message:
{“code”:”rest_no_route”,”message”:”Geen route gevonden die overeenkomt met de URL en aanvraagmethode.”,”data”:{“status”:404}}

How can I solve this 404 error? Because I think that is the problem why the focus keywords doesn’t get saved.