It's the only post doing it as far as I can tell. Looking over the logs via Xenforo and through the cpanel there is no errors."not implemented" is http 501. That is something from the web server itself, not XenForo.
Does this happen on all posts? Is the error consistent within that one thread?
Basically you need to contact your web host about this. Ideally the server logs will contain more information about the error.
That sounds suspiciously like a mod_security rule that is sensitive to a keyword in the referrer. For example, it might be sensitive to "ps" in the URL because that is a common linux command.It's the only post doing it as far as I can tell. Looking over the logs via Xenforo and through the cpanel there is no errors.
Looking into the way XenForo Formats the Thread's URL - A Mod_Security rule would indeed block the post to that thread or any other thread because it's URL has "/ps-" Which in some way or another is a way of a Web Attack Injection method according to a Mod_Security.
Going deeper into the issue brought me to a Drupal issue which happens on a POST http Command as well using "ps" in the URL, They experience an Apache 403 Forbidden but XenForo has it's own error handling for customization within the forum hence why we see it as "Not Implemented" With the command it was attempting to use "POST"
ps is a unix command for showing processes, We in the datacenter environment utilize that command to see what processes are running on a system, and shows resource usage as well.
The Post I found: https://www.drupal.org/node/164989
It would be a good point in direction as you would not be accessing any "ps" command on the server as you are on a shared system, with that being said we can always attempt to whitelist "ps" to see if the error triggers the mod_security rule.