Error 619 PPTP VPN on Windows 7

I have spent 3 hours trying to work this out but I'm all out of ideas.

I have a 3G internet connection via a USB dongle and a Windows 7 laptop, I cannot connect to a PPTP VPN while connected to the 3G network.

I keep getting error 619.

When I use a wireless network on this same laptop I can connect to the VPN fine.

I have tried the same 3G modem in a Windows XP machine and I can also connect to the VPN just fine.

So the problem seems to be the 3G connection and Windows 7.

I have tried lowering the MTU, disabling the firewall, enabling everything VPN related in the firewall, disabling antivirus and antimalware and disabling the internal modem.

Has anyone else come across this problem?

I would really appreciate any feedback at all.

Thanks in advance.