I was having problems getting PPTP to work with PureVPN on a Lubuntu VM. I followed PureVPN's directions, but only one of the four VPNs I created actually worked. The configuration appeared to be identical, based on what the user interface was telling me.
When I looked at the VPN configuration files in /etc/NetworkManager/system-connections, I found a discrepancy. The VPN that worked contained three entries under [vpn] that were missing from the other files:
[vpn] refuse-chap=yes refuse-eap=yes refuse-pap=yes
Once I added these entries, the PPTP connections worked as expected.