Comment 10 for bug 346768

And by default the pptp driver is not loaded:

$ lsmod | grep pptp

it prints nothing.

So have to load manually:

$ sudo modprobe nf_nat_pptp
$ sudo modprobe nf_conntrack_pptp
$ lsmod | grep pptp

it prints now:

nf_nat_pptp 2844 0
nf_conntrack_pptp 5984 1 nf_nat_pptp
nf_conntrack_proto_gre 5344 1 nf_conntrack_pptp
nf_nat_proto_gre 2080 1 nf_nat_pptp
nf_nat 17808 2 nf_nat_pptp,nf_nat_proto_gre
nf_conntrack 67608 5 nf_nat_pptp,nf_conntrack_pptp,nf_conntrack_proto_gre,nf_nat,nf_conntrack_ipv4

But anyway, it still doesn't work.