Ok looks like they've suddenly enabled TLS1.2 on it which wasn't a requirement before.
Fixed my powershell script by placing the below before the Invoke-WebRequest in-case it's any use to anyone else:
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
Zoneedit DYN did have a brief outage stemming from server upgrades on the 18th that carried over to the 19th. The issue was fixed around noon (EST) on the 19th, so all access should have been restored since then.