I had a few problems connecting to a client's VPN. All the settings were correct:

Configuration: Default
Server Address: vpn.example.com
Account name: mrn
Encryption: Automatic

but it would not connect, it always timed out.

/var/log/ppp.log:

publish_entry SCDSet() failed: Success!
publish_entry SCDSet() failed: Success!
pptp_get_router_address
pptp_get_router_address 192.168.0.1 from dict 1
PPTP connecting to server 'vpn.example.com' (x.x.x.x)...
PPTP connect errno = 60 Operation timed out

I came across this forum post that said it ended up being the router port forwarding stopped for the VPN port, so I checked my Virgin Super Hub router. After logging in go to the "Advanced Settings", "Security", "Firewall" section. I checked "PPTP Pass-Through" to enable forwarding of PPTP traffic, applied the changes and that fixed it.