Easy way: Confiared can provide paid support to help you. It's not free, but you have professional SysAdmin to fix your server.
Hard way: You can follow the next points to debug your self you server
Check the IPv6 part
Check if you have configured IPv6 AAAA/CNAME entry in you DNS and it's on 2803:1920:XXXXXXX
Test via ping if it resolv on IPv6 address too (not only IPv4)
Test connectivity on IPv6, if you don't have IPv6, you can use our proxy: sockproxyipv6.confiared.com:1080
Check if from any 2803:1920:XXXXXXXXX you can access to you web server, you can you the proxy above
Other part
Maybe the request is too long, to respect the mutualised ressources of reverse proxy, the request time without data is limited in time. You can bypass using IPv6
Maybe you server return error, then reverse proxy to prevent informations leak from any body, drop the error page