This is one of the most annoying problems i ran into and it also caused  ISA server RDP issues as well.
After many calls to microsoft and countless hours here was the cause of my error;
1. the lowest numeric public ip on the firewall must be the primary IP on the public NIC;
so if you have ;
70.69.71.60-80 address range on the NIC, then the IP listed as the primary IP when you view the NIC properties must be
70.69.71.60 (not 70.69.71.66, etc...)
2. If you have more than 1 subnet like I did, you have to use the lowest IP of all the subnets...
(ex; 
   subnet 1; 70.69.71.60
   subnet 2; 67.123.05.41
   the primary IP would have to be 67.123.05.41)

to make sure RDP works in this case, RDP should be configured to listen only on the internal NIC.