I don't find any logs for process -> pppd. Searching racoon I find successful connections. In the server logs, I don't see anything that stands out...
Mon Sep 26 21:28:41 2016 : sent [IPCP ConfReq id=0x1 <addr 199.19.xxx.xxx>]
Mon Sep 26 21:28:41 2016 : sent [ACSCP ConfReq id=0x1]
Mon Sep 26 21:28:41 2016 : rcvd [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Mon Sep 26 21:28:41 2016 : ipcp: returning Configure-NAK
Mon Sep 26 21:28:41 2016 : sent [IPCP ConfNak id=0x1 <addr 10.0.0.151> <ms-dns1 10.0.0.1> <ms-dns3 10.0.0.1>]
Mon Sep 26 21:28:41 2016 : rcvd [LCP ProtRej id=0x2 82 35 01 01 00 04]
Mon Sep 26 21:28:41 2016 : rcvd [IPV6CP ConfReq id=0x1 <addr fe80::6aa8:6dff:fe00:06b6>]
Mon Sep 26 21:28:41 2016 : Unsupported protocol 0x8057 received
Mon Sep 26 21:28:41 2016 : sent [LCP ProtRej id=0x2 80 57 01 01 00 0e 01 0a 6a a8 6d ff fe 00 06 b6]
Mon Sep 26 21:28:41 2016 : rcvd [IPCP ConfAck id=0x1 <addr 199.19.xxx.xxx>]
Mon Sep 26 21:28:41 2016 : rcvd [IPCP ConfReq id=0x2 <addr 10.0.0.151> <ms-dns1 10.0.0.1> <ms-dns3 10.0.0.1>]
Mon Sep 26 21:28:41 2016 : ipcp: returning Configure-ACK
Mon Sep 26 21:28:41 2016 : sent [IPCP ConfAck id=0x2 <addr 10.0.0.151> <ms-dns1 10.0.0.1> <ms-dns3 10.0.0.1>]
Mon Sep 26 21:28:41 2016 : ipcp: up
Mon Sep 26 21:28:41 2016 : found interface vlan0 for proxy arp
Mon Sep 26 21:28:41 2016 : local IP address 199.19.xxx.xxx
Mon Sep 26 21:28:41 2016 : remote IP address 10.0.0.151
Mon Sep 26 21:28:41 2016 : Received protocol dictionaries
Mon Sep 26 21:28:41 2016 : Received acsp/dhcp dictionaries
Mon Sep 26 21:28:41 2016 : Committed PPP store
Mon Sep 26 21:28:41 2016 : Received acsp/dhcp dictionaries
Mon Sep 26 21:28:41 2016 : Committed PPP store
Mon Sep 26 21:28:41 2016 : l2tp_wait_input: Address added. previous interface setting (name: en0, address: 199.19.xxx.xxx), current interface setting (name: ppp0, family: PPP, address: 199.19.xxx.xxx, subnet: 255.255.255.0, destination: 10.0.0.151).
Mon Sep 26 21:29:33 2016 : rcvd [LCP TermReq id=0x3 "User request"]
Mon Sep 26 21:29:33 2016 : LCP terminated by peer (User request)
Mon Sep 26 21:29:33 2016 : ipcp: down
Mon Sep 26 21:29:33 2016 : sent [LCP TermAck id=0x3]
Mon Sep 26 21:29:33 2016 : l2tp_wait_input: Address deleted. previous interface setting (name: en0, address: 199.19.xxx.xxx), deleted interface setting (name: ppp0, family: PPP, address: 199.19.xxx.xxx, subnet: 255.255.255.0, destination: 10.0.0.151).
Mon Sep 26 21:29:33 2016 : L2TP received CDN
Mon Sep 26 21:29:33 2016 : Connection terminated.
Mon Sep 26 21:29:33 2016 : Connect time 0.9 minutes.
Mon Sep 26 21:29:33 2016 : Sent 10433 bytes, received 57601 bytes.
Mon Sep 26 21:29:33 2016 : L2TP disconnecting...
Mon Sep 26 21:29:33 2016 : L2TP disconnected
2016-09-26 21:29:33 PDT --> Client with address = 10.0.0.151 has hungup