[SOLVED] Error accessing uncompiled pages using ISA Server 2006 SP1

Error accessing uncompiled pages using ISA Server 2006 SP1

We are in the processing of configuring a portal to use ISA Server as our front end security provider.
So we are using ISA Server 2006 SP1.

Unfortunately when we access .net applications through ISA Server, the first time they are accessed.
i.e. They are not compiled yet, the following error appears:
Error Code: 500 Internal Server Error. The parameter is incorrect. (87)

In the ISA Monitoring logs, this shows:

Failed Connection Attempt
Log type: Web Proxy (Reverse)
Status: 87 The parameter is incorrect.

Once the application is compiled, the error never appears.
Does anyone know how to resolve this, so the site works correctly the first time?

Some additional information:

The websites accessed are running on windows server 2008 64 bit - standard edition, and occurs for Sharepoint as well as standard .net websites.
ISA Server is running on Windows server 2003 R2 SP2 Standard eidtion
The firewall on the windows server 2008 box allows all access. (To rule this out.)
Nothing odd appears in the IIS logs or firewall logs.

Solution

This problem was caused by the Authentication Delegation value in ISA Server being set to NTLM while IIS was setup to accept Windows Authentication. This combination appears to be a problem in my environment.

Changing it to basic authentication, or any other valid combination works correctly, thus I'm going with a different authentication delegation setting.