I've reproduced this xl2tpd issue with openSUSE Leap 15.0, even though it is running kernel-4.12.14-lp150.12.4-default. So I guess something got backported from a newer kernel which now breaks xl2tpd. It would be great if Leap 15.0's xl2tpd could also be updated to version 1.3.12 or the aforementioned commit/patch be applied to the existing xl2tpd-1.3.10.