VPN Problem - Error 721

I have an SBS 03 with an ETEC Wireless ADSL Modem Router.
I have opened ****t 1723 on the Firewall in the router and forwarded it to 
192.168.16.2 (Server NIC).
When attempting to VPN from a Client I get error 721.
The firewall logs that it passed the packets through correctly but the 
server will not allow authentication. I have checked every setting on the 
server many times against MS Kbase articles and all is OK