Sometimes PPTPd would work and sometimes won’t.
When you see in the logs:
GRE: xmit failed from decaps_hdlc: Operation not permitted CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
We tell iptables to open the INPUT chain of the filter for the GRE protocol.
iptables -t filter -A INPUT -p gre -j ACCEPT