Hi Stuart,
just my 2 cents for discussion.
I don't think that you have an issue with the SSL Offloading because the call is being done in HTTP and not HTTPS.
Also, you don't have any certificate errors trying to call an application webpage.
Are you using Lifetime? If so, in the 'Infrastructure' - 'Environment Security', 'Force HTTPS for screens...' option enabled?

Could be something related to this.
Let me know the culprit of this mistery!