When dialling a PPTP VPN, the dial up fails and hangs on verifying username and password. It then produces an 806 error.
In my case, port 1723 was open and i could prove this by viewing the router and by telnet to the IP and port 1723. The issue stood out as an authentication problem as this is where the process hangs.
The solution was to enable “Routing and Remote Access (GRE-In) on the domain profile of the RRAS server.
No idea how this turned itself off, but it did.