I found a fix for users with a Netgear C3000 cable modem router that are unable to connect to PPTP VPN.
In my test case, the user was on Comcast and getting error 800 when connecting to her company VPN.
First, make sure you can log into the router. Default login and password are admin/password. You'll need to be able to log in.
Netgear has this page which gives a download which makes an update to the router firmware to allow VPN passthrough:
In my test, I had to apply the update and then manually reboot the router, but afterwards, I was able to connect to the PPTP VPN per normal.