callback url not being processed

Is the website using CSRF protection? If so, that is the reason for the callback not running. The 3rd party is calling the URL without the token and thus the page call will fail.

Thanks Martin for the suggestion. I had to look it up ...

So I checked the installed Apache2 modules and CSRFProtector is not installed.
I believe we have a vanilla Ubuntu 14.02 so unless there is something else within the OS I'm pretty sure this is not the cause.

Any other ideas please?

Or just in case how would I enable an exception?

Cheers