Comment 16 for bug 1568560

There is a fix for this:
https://ubuntuforums.org/showthread.php?t=2305819

add a file to the /etc/modules-load.d directory to have the modules loaded into the kernel at boot. I just created a file called netfilter.conf and put the following in it:

nf_nat_pptp
nf_conntrack_pptp
nf_conntrack_proto_gre

Is it safe to use this solution? I don't know what those three entries mean.