CRM IFD Access Restricted for 2 Users. Receiving 404 Errors

We have recently deployed CRM2011, upgrading from 4.0, in doing so there are 2 users that cannot access our external URL set up through IFD.

Both users can access CRM using internal and external URL ONLY when on network. However, I am able to access using my credentials from their workstations off-network.

When they attempt to log in using IFD, they submit their credentials via ADFS log-in, and then receive a CRM 404 error.

I have checked all their setting in Active Directory and things look good.

Any ideas as to what could be causing this?