I am receiving event 789 when connecting to my Forefront TMG VPN server from windows 7 clients via L2TP. This does not occur on XP systems. I've run through the debugging at http://blogs.technet.com/b/rrasblog/archive/2009/08/12/troubleshooting-common-vpn-related-errors.aspx without success.
I'm receiving error 4976 server side with the following message. "During main mode negotiation, IPsec received an invalid negotiation packet." How can I resolve this?