A client called our office this morning and said "My users can't access the Internet or browse shared folders!". The client runs Windows SBS 2003. RRAS w/ NAT distributes Internet access on the WAN interface to all the workstations.
What was wrong? File and Printer Sharing had been disabled on the LAN interface (and enabled on the WAN)! This generated lots of errors in the event log, including event 1058 and 1030, because Windows couldn't access the SYSVOL share via UNC path (in fact, while shares were visible via \\servername, double-clicking on any of them merely brought up a username/password prompt). After correcting this, users could once again access shared resources on the server, but they still couldn't access the Internet.
It turned out that the LAN interface had recently been replaced or renamed...so RRAS wasn't doing NAT on the renamed interface. Right-clicking in RRAS/NAT and choosing "Add interface" quickly resolved the problem.