ISA - Local address blocked

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We have ISA configured with 2 site and content rules.

Rule 1: Intranet users - Configured to all External address denied. We
have added the group intranet only users to this content rule

Rule 2: Web Access - Configured to all external addresses enabled. We have
proxy users group applied to this rule.

Everything appears to be fine until we access 1 particular web application.
This web application has its own credentials and does not rely on windows
credentials. It has a SQL back end. We can reach the log in page fine.
When we log in the page changes to the main application page but we are then
prompted by our ISA server for credentials. When we hit cancel a couple of
times the promp disapears.

Any ideas?
.