[8.1 / 8.4] Accessing HAP through Forefront TMG 2010

Hello,

As part of our plan to upgrade some old systems, we have replaced ISA 2000 and myClasslink with Forefront TMG 2010 and HAP+.
Here is the problem im having...

Accessing the test hap install (8.4) from internally (which will ignore TMG and go straight to the horses mouth) and externally using a different directory (Production HAP is /xtra/, Test HAP is /xtra_test/. Both ASP.NET 4 Applications in the HAP pool.) works absolutely fine. This is the sight from the test directory externally.
Attachment 16228
All working fine as you can see. Now if I goto the production site... Remember: This is the SAME directory & files that IIS is serving you.
Attachment 16227
Not working at all, and all the difference is the not working url is https://bla.bla.bla/xtra/, where the working URL is https://bla.bla.bla/xtra_test/.
Both the same files from the same folder.

Any clues?

Perhaps if the title read "Possible IIS Misconfiguration" It would have gained more attention.