L2TP/IPsec VPN Failure

Hello all, I've just got a SM-G920F, Android 5.1.1 (kernel 3.10.61-5012324), and I am experiencing a problem with VPN connect to a Linux box that is running libreswan. This box works wonders with an iphone 5 and a Nexus 7. With the S6 i can track the pluto activity on the server and phase 1 and 2 correctly done, but after the IP assignment the l2tpd never gets called, and after a while the phone signals connection failed. Any idea about this beahviour? Suggestions about what to analyze in logcat ?

regards
Mat