Hey I got one,
I am trying to set up a VPN connection and am recieving this 721 error. Here is the situation. I have (2) PC's, seperate accounts and both users have dial in access. Both are connected through a DLINK DSL-G604T and have internet access. Both machines are running XP with the latest service packs and updates. One PC can VPN and the other cannot. Thinking that it was a problem with having multiple VPN clients through the router, I disconnected the first and tried to VPN on the second. I still receive error 721. I then tried the other(working) username which connected on the first machine and the problem was the same. Both accounts have known dial in access and work. Having even tried disabling Windows Firewall with no solution. Both VPN connection settings are identical. It is just that on this one PC I cannot VPN in, no matter which account I use I still receive Error 721 not responding. Looking into the event viewer, on the responding server (Win2K3) a message displays something about the remote firewall. I do not think that it is because I can VPN on one PC and not the other.
" A connection between the VPN server and the VPN Client XXX.XXX.XXX.XXX has been established, but the VPN connection cannot be completed. The most common cause for this is that a firewall or router between the VPN server and the VPN Client is not configured to allow GRE packets (protocol 47). Verify that the firewalls and routers between your VPN server and the internet allow GRE Packets. Make sure the firewalls and routers on the users network are also configured to receive GRE packets. If the problem persists, have the user contact the ISP to determine wether the ISP might be blocking GRE packets"
I do not think that the above information is the solution.
So in summation, I have (2) PC's attached to a DLS Modem/firewall/router. PC1 can VPN in with no problems, PC2 cannot. VPN connection settings are the same.