When a user tries to connect using Windows XP, he gets as far as registering computer on the network, then: TCP/IP CP reported error 733: A connection to the remote computer could not be completed. You might need to adjust the protocols on this computer.
On the server, Event 20050 occurs, saying The User DOMAIN\username connected to port VPN4-23 has been disconnected because no network protocols were successfully negotiated.
These errors occur whether we are attempting a connection from internal, or external, so it does not appear to be firewall related.
We updated a few nights ago, 4/22/2008, applying all Windows updates from the last month.
Prior to doing this, the VPN connection itself was working great. We thought this was the culprit.
We have uninstalled all updates applied 4/22, and no change in the results.
Server: Windows 2003 Enterprise R2
Clients: Windows XP Pro, Windows Vista Business
What troubleshooting steps should I take?