After allowing two seemingly inoccuous Windows Updates (KB2388210 and KB982632) our production Windows Small Business Server 2003 (SP2) required a reboot.
After the reboot all VPN connections failed with 691.
Things I have confirmed so far:
The Password is being verified as there is a different error with a wrong password.
The server EventLog entry says "The user [domain]/[username] has connected and failed to authenticate on port [VPNport]. The line has been disconnected.".
The router and other external infrastructure is not relevant as we can confirm the same issue trying to VPN from a machine on the local network.
We believe our "Accounting Provider" was always None, but changing it to Windows didn't seem to change any thing.
We do have ISA sticking its nose in, and attempting to reset VPN settings from there changed nothing.
(This has been "cross-posted" to the Microsoft SBS Forum)