You've seen it before. Someone is having a problem with her VPN connection and the first suspect is the ISA Firewall. The problem is that everyone else has no problems connecting to the VPN server through the firewall. Sort of makes it hard to argue that the ISA firewall is at fault. As I've noted here many times, 99.87% of the time the problem is not related to the ISA or TMG firewall.
So what to do? What if you have a connect to a PPTP VPN server and it just won't work from behind the firewall. Is it the firewall's fault? Is it the user's fault? Is it somebody else's fault?
Most likely it's Linux fault. If you are using a NAT device based on a Linux operating system (you mean hardware uses operating systems????), there's a good chance that their PPTP NAT editor is horked.
For all the details and some suggested fixes, check out the ISA/TMG Firewall Team blog over at https://blogs.technet.com/isablog/archive/2009/01/07/a-pptp-client-might-fail-to-connect-to-a-vpn-server-on-the-internet-through-an-isa-server-2006.aspx
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer