Microsoft ISA PPTP VPN - Suddenly no users can connect to VPN (619 and 806)

Hello;

In an SBS 2003 Enterprise environment running ISA Server as the firewall and VPN endpoint, as of yesterday mid-day, suddenly no users can connect to the VPN (using the Windows built-in PPTP VPN client.)  I know that ISA server can become flaky with respect to VPN connectivity, and I'm already working on getting this organization to migrate to something a little more stable (such as a Cisco ASA or WatchGuard).  In the meantime, I'm having to troubleshoot this issue.

The error users get when remotely trying to connect are either error code 619 or 806, and the VPN connection hangs at "verifying username and password".

Many times in similar cases, I've either recycled the ISA related services or rebooted the ISA server entirely and the problems have gone away.  This time this isn't working.

I also know that many times these error codes are a problem with PPTP or GRE being passed through one of the routers on either end.  In this case, the router/firewall on the end of the VPN endpoint IS the firewall of course.

Here's a list of what I've done so far.  

 As of some time today, no users can VPN into SBS.
* Recycled ISA related services.  Did not fix.
* Rebooted ISA/SBS server.  Did not fix.
* Tested from mulitlple outside locations--does not work from any.
* Reviewed VPN and firewall policies on ISA server.  No problems found.
* Reviewed event logs, no errors relating to ISA VPN found.
* Tested PPTP port inbound from canyouseeme.org.  Port is open, this is not the problem.
* Created a test policy allowing all VPN-related protocols inbound, did not fix.
* Power cycled cable modem @ server side location.  Did not fix.
* Used pptpclnt and pptpsrv utilities to test passing PPTP and GRE traffic--successfully transferred.  (So does not appear to be an ISP protocol or port blocking issue.)
* Verified server is in RAS and IAS Servers active directory group.
* Tried enabling all protocols on SBS outside NIC--did not fix.  
* Researched VPN errors 619 and 806; no additional information found.
* Reviewed list of installed MS patches for one that has been known to break ISA VPN--not installed.
* Reviewed event logs after second reboot to see if any new event have been logged--none found.
* Reviewed Dell OpenManage for any signs of hardware trouble--none found.

I'm out of ideas!  Anyone have any suggestions?