Just restart or stop the ALG service on the NAT box, yes it is as simple as that.

After two days searching the Internet and trying to figure out the patch the causes this issue, found nothing related to RRAS or natting. However, by chance founded the ALG relationship with FTP access. ALG provides a hidden layer for applications doesn’t support proxy option to allow it to access the Internet. But FTP has it’s own buit-in filter in Windows Server. Which is affected in somehow by the ALG/NAT combination.