Clients receives error 691 trying to connect to ISA Server 2006 as VPN Server

Yikes! Not another 691 error on a VPN client connecting to the ISA firewall. What’s the cause this time?


If you look at the error, you’ll think the problem is an authentication issue, since that what the dialog box says it is 🙂

Yuri Diogenes goes into deep detail in how to troubleshoot these kind of issues in his article over at

Make sure to have a strong cup of coffee before reading this – it’s a very deep dive.



Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer

Prowess Consulting

PROWESS CONSULTING | Microsoft Forefront Security Specialist
Email: [email protected]
MVP — Forefront Edge Security (ISA/TMG/IAG)

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