Unable to contact support, presumably because there are free domains on my account, but this is a paid Pro domain. Have emailed sales, who have promised to contact support, but almost a week has gone by and no contact. Nothing wrong with the origin server. Some domains on CF work, some don’t.
The 522 error you are getting is because you are not allowing all Cloudflare IPs at your origin server. To stop this error from occurring you should allow all IPs from Cloudflare to reach your origin using port 443 for HTTPS connections. You can find a list of all Cloudflare IPs here: IP Ranges