Can you show what results you get? If you have just changed the nameservers for your domain to Cloudflare it may be that some resolvers still return the previous nameservers until the TTL expires.
Likely you need to set the DNS records to “DNS only” and not proxied so they can see their IP addresses instead of the Cloudflare proxy ones.
This will mean all requests go direct to your host and not through Cloudflare. You can try re-enabling the proxy after they have validated but if you get an error in 3 months you’ll need to do it again so they can renew the cert.