pptpd needs nf_conntrack_pptp loaded and ip forwarding enabled to work.

So I propose to add a following two files:

0. Something like /usr/lib/modprobe.d/pptpd.conf with the following content:

# Load nf_conntrack_pptp.ko at boot
nf_conntrack_pptp

Actually there might be a better place - put it to 
/usr/lib/modules-load.d/ (see man modules-load.d).

1. Something like /usr/lib/sysctl.d/20-pptpd.conf with the following content:

# Enable IP forwarding
net.ipv4.ip_forward = 1

W/o that this package is useless, and it's not simple to find outr what's going on wrong.