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 https://blogs.technet.com/isablog/archive/2009/03/19/clients-receives-error-691-trying-to-connect-to-isa-server-2006-as-vpn-server.aspx
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