Disclaimer: I don't run L2TP/IPSec, so this may be unintentionally misleading.
There are several more modern "howto" documents floating around the Internet since the first 2012 article on L2TP/IPSec was published at the OpenBSD Journal.
This one recommends
enc "3des" for your ipsec.conf, and therefore there may be something to it, as isakmpd(8) complains that it was expecting 3DES_CBC in your log.