Created attachment 108348 [details] Network Connections under Unity Under L2TP IPSec Options in Kubuntu 17.04, there's a field of Pre-shared key (only). In Ubuntu 17.04, there are Advanced fields for Phase1 Algorithms and Phase2 Algorithms. Please refer to the attached images. Under Ubuntu 17.04, I can configured and connect the IPsec tunnel to the L2TP host. Under Kubuntu 17.04, I can't.
Created attachment 108349 [details] KDE L2TP IPSec Options
FYI, the configuring that led me to this problem is described at https://ingbrief.wordpress.com/2017/10/14/ubuntu-l2tp-vpn/ .
The options you are looking for were added in Plasma 5.11.