Hello.
As far as I can remember, there is an issue with pptp. Actually it is not a bug, but some default value in how the connection is handled (don’t blame me, I’m not an expert), and not strictly a firewall rule.
The solution was to issue this command:
sudo sysctl net.netfilter.nf_conntrack_helper=1
Then retry to connect.
If it works, let’s set it permanent, in order to survive to reboots:
echo net.netfilter.nf_conntrack_helper=1 | sudo tee /etc/sysctl.d/98-nf_conntrack.conf