I've used the patch provided and rebuilt the deb on 4.15 and can confirm this fixed the udp error message. Using PAP auth w/ ipsec works again, however can't confirm other possible regressions beyond my usecase.
- Bug#894674: kernels 4.15 and 4.16 break xl2tpd Douglas Kosovic
- Bug#894674: (no subject) Max Resnick
