Get a list of IP addresses used by cloud flare to access your server and whitelist them. If you are using shared hosting then get your host to white list the IP addresses as it is most likely a firewall issue.
This could also be caused by one of the plugins that you may be using to protect your forum from spam. If you have a plugin that required certain amount of time between two requests that could also be causing this.
There is another scenario I can think of and its easy to reverse if that is the case:
If you may have hit that built in feature of xenforo where a user would get delayed page loads and you did that to an IP from cloudflare then that could be causing this error as well. This would only happen if you havent done any modification to show the real IP of a user and every user shows up with cloudflare IP.
These are the only ones that I can think of right now .