For people struggling with this issue, another workaround is to access the forum server IP address directly, instead of going through CloudFlare. Normally, that IP address should be secret, and it should have a firewall configured to block accesses from any source other than CloudFlare, to prevent spammers (etc) from circumventing CloudFlare. So, when you use this method, you might have to temporarily open the firewall, and then PM the URL with IP address in it to the person having a problem so they can make their important post, and then you would close up the firewall again.
You can't unsecret the IP once you share it, but if you have a firewall appliance ahead of the server itself, and it's configured properly, it's not really necessary to keep the IP secret. In that case, you would open the firewall explicitly for the IP of the person you're helping, and no others. I HIGHLY recommend pfSense as your dedicated firewall.
If this continues to be a problem, I recommend switching to Incapsula. It handles forums a bit better than CloudFlare. It's more expensive, but you get what you pay for.
Hope this helps.