Trouble with error 721 trying to make a VPN connection to Small Business Server

I am yet another user getting the error 721 when trying to make a VPN connection from my Windows XP Pro (SP2) machine to a Small Business Server 2003 VPN Server. I am using a Linksys router through a cable modem back through a cable modem to another Linksys router at the server. Both routers pass through PPTP and open up port 1723. The user is configure to allow remote connections.

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It seem to me that the problem is coming from the Linksys on all of these questions.  Seems like it won't pass the GRE protocol which is going to be required for a PPTP VPN.  I'm not sure if anyone else has come up with any ideas relating to this.
I would try test it internal. If it works, then try to use DMZ on the router. If the DMZ doesn't works, reset the router. More more information, check
Tim HolmanCommented:
Upgrade the Linksys firmware in both instances - there are known issues with older versions.  
What model Linksys is this anyways ?

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steve_webberAuthor Commented:
I was independently updating to the latest firmware. I am awarding the points to this suggestion because that is what worked and by indicating this as the accepted solution, others may benefit.

Well actually, until now I am using the updated firmware version (1.04.05) and still "Seems like it won't pass the GRE protocol which is going to be required for a PPTP VPN".

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