Any other page on the domain is working as it should be. Also, when I disable the CDN and only use Cloudflare DNS services, the page seems to work fine.
Any help or indication about the right direction would be highly appreciated as that particular page is also the page that’s bringing 90% of the website’s traffic and revenue.
that particular page is also the page that’s bringing 90% of the website’s traffic
Above information suggests it is possible you have a solution which bans requests more than a limit (rate limiting) to certain URLs when it comes from the same IP (CF network).
I’ve checked that article and checked the error logs for the server but nothing seems wrong here. That’s why I’ve tried reaching out to this community to see if anyone has a better idea.
Also the page working with the default DNS (no proxy) makes me thing this could be a compatibility issue rather than a webserver issue.
After disabling all the plugins and switching back to the default Wordpress theme, the result seems to be the same. I thought so first about the firewall (Worfence/Server) but disabling the security options doesn’t seem to bring any result.
If you have tried anything in that article, and what @Xaq suggested, I don’t think there is anything else I can suggest - only to contact support with two HAR files, one with Cloudflare enabled and the error page and one with Cloudflare disabled when it works to see if they can track it down!
Sorry!
If you do, please post back and let us know how you are getting on.