Sonicwall VPN users, authenticated by NPS on an SBS 2011 server, were getting "Error 812" when they tried to connect - "The connection was prevented because of a policy configured on your RAS/VPN server..."
The Security Event Log on the server reported Event ID 6273 - Reason Code: 65. The Network Access Permission setting in the dial-in properties of the user account in Active Directory is set to Deny access to the user.
This was resolved by deleting and recreating the Network Policy in NPS.
