Ok, a bit of a headscratcher. First of all, I didn’t realise that UFW was blocking port 1723 traffic, which caused all VPN connection attempts to persistently fail - proof was to be found in /var/log/syslog and was easily remedied:
Step 1: Enable Port 1723 Link to heading
sudo ufw allow 1723
Step 2: Allow GRE Protocol Link to heading
However, this still didn’t resolve the issue. And it wasn’t until some searching that I came across this article that I finally resolved the problem by following option 1 (adding -A ufw-before-input -p 47 -j ACCEPT to before.rules).
Add the following line to /etc/ufw/before.rules:
-A ufw-before-input -p 47 -j ACCEPT
Originally published at https://jensenmo.blogspot.com/2015/03/getting-pptp-vpn-in-ubuntu-working-1404.html