We have been using Web Helpdesk for a couple of months now in our organisation with no problem.
I have now published the helpdesk to the internet via a reverse-proxy server hosted in our DMZ (Application request routing as part of an IIS 8 deployment). This forwards requests to helpdesk.domain.com through our Firewall and on to the internal server. The proxying appears to be working mostly alright as users can log into WHD and view tickets etc. WHD is accessed using the same URL both internally and externally so that we have a consistent address and certificate being presented to our users, our DNS servers translate the DNS name to either local or public IP depending on location.
Any ideas what would be causing this change in behaviour between internal and external users?