I was having issues setting up the cited "network-manager-l2tp" client and I managed to solve this.
Apparently, recent releases of Ubuntu won't allow to complete a L2TP/IPsec handshake because of 3des.
When setting the Phase1 and Phase2 algorithms, add an exclamation mark at the end.
Phase1 algorithms: 3des-sha1-modp1024!
Phase2 algorihms: 3des-sha1!
Apparently, this forces the client to use insecure ciphers and therefore the handshake can proceed.