Odd cURL Error - errno 104 (Code 56) In WHMCS
Finally worked out what it was. Turns out that in a bid to make my server as secure as possible I had used the Host Access Control to limit access to the whostmgrd service to only certain known IPs. I hadn't thought to put the IP of the server itself which caused it to fail when WHMCS started trying to make API calls.
Only a day and a half of my life wasted. What fun!