Connecting Mac OSX to DrayTek Vigor 3900 - L2TP-VPN server did not respond. Try reconnecting.

L2TP-VPN server did not respond. Try reconnecting. If the problem continues, verify your settings and contact your Administrator.

I have been through the settings at: https://www.draytek.com/en/faq/faq-vpn/vpn.host-to-lan/how-to-establish-vpn-connection-from-mac-os-to-vigor3900-via-l2tp-over-ipsec/

But still receive the same message - it is obviously contacting the server but not connecting for some reason. I have double/triple checked the passwords and IPsec pre-shared keys but no good - any ideas anyone?

Firmware on 3900 is the latest...

Log file from Mac is as follows:

Thu Aug 24 17:29:15 2017 : IPSec connection started
Thu Aug 24 17:29:15 2017 : IPSec phase 1 client started
Thu Aug 24 17:29:15 2017 : IPSec phase 1 server replied
Thu Aug 24 17:29:17 2017 : IPSec phase 2 started
Thu Aug 24 17:29:17 2017 : IPSec phase 2 established
Thu Aug 24 17:29:17 2017 : IPSec connection established
Thu Aug 24 17:29:17 2017 : L2TP sent SCCRQ
Thu Aug 24 17:29:20 2017 : L2TP received SCCRP
Thu Aug 24 17:29:20 2017 : L2TP sent SCCCN
Thu Aug 24 17:29:20 2017 : L2TP sent ICRQ
Thu Aug 24 17:29:20 2017 : L2TP received ICRP
Thu Aug 24 17:29:20 2017 : L2TP sent ICCN
Thu Aug 24 17:29:20 2017 : L2TP connection established.
Thu Aug 24 17:29:20 2017 : L2TP set port-mapping for en0, interface: 4, protocol: 0, privatePort: 0
Thu Aug 24 17:29:20 2017 : using link 0
Thu Aug 24 17:29:20 2017 : Using interface ppp0
Thu Aug 24 17:29:20 2017 : Connect: ppp0 <--> socket[34:18]
Thu Aug 24 17:29:20 2017 : sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x7dd8ce14> <pcomp> <accomp>]
Thu Aug 24 17:29:20 2017 : rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <auth chap MD5> <magic 0xb28db77>]
Thu Aug 24 17:29:20 2017 : lcp_reqci: returning CONFACK.
Thu Aug 24 17:29:20 2017 : sent [LCP ConfAck id=0x1 <asyncmap 0x0> <auth chap MD5> <magic 0xb28db77>]
Thu Aug 24 17:29:23 2017 : sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x7dd8ce14> <pcomp> <accomp>]
Thu Aug 24 17:29:23 2017 : rcvd [LCP ConfRej id=0x1 <pcomp> <accomp>]
Thu Aug 24 17:29:23 2017 : sent [LCP ConfReq id=0x2 <asyncmap 0x0> <magic 0x7dd8ce14>]
Thu Aug 24 17:29:23 2017 : rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <auth chap MD5> <magic 0xb28db77>]
Thu Aug 24 17:29:23 2017 : lcp_reqci: returning CONFACK.
Thu Aug 24 17:29:23 2017 : sent [LCP ConfAck id=0x1 <asyncmap 0x0> <auth chap MD5> <magic 0xb28db77>]
Thu Aug 24 17:29:24 2017 : L2TP port-mapping for en0, interfaceIndex: 0, Protocol: None, Private Port: 0, Public Address: 0, Public Port: 0, TTL: 0.
Thu Aug 24 17:29:24 2017 : L2TP port-mapping for en0 inconsistent. is Connected: 1, Previous interface: 4, Current interface 0
Thu Aug 24 17:29:24 2017 : rcvd [LCP ConfAck id=0x2 <asyncmap 0x0> <magic 0x7dd8ce14>]
Thu Aug 24 17:29:24 2017 : sent [LCP EchoReq id=0x0 magic=0x7dd8ce14]
Thu Aug 24 17:29:24 2017 : rcvd [CHAP Challenge id=0x42 <825c0b37b659fec502f69ef2b4aae467>, name = "xl2tpd"]
Thu Aug 24 17:29:24 2017 : sent [CHAP Response id=0x42 <44761826d41079d83b09c9076a732b1d>, name = "username"]
Thu Aug 24 17:29:24 2017 : rcvd [LCP EchoRep id=0x0 magic=0xb28db77]
Thu Aug 24 17:29:24 2017 : rcvd [CHAP Success id=0x42 "Access granted"]
Thu Aug 24 17:29:24 2017 : CHAP authentication succeeded: Access granted
Thu Aug 24 17:29:24 2017 : sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Thu Aug 24 17:29:24 2017 : sent [IPV6CP ConfReq id=0x1 <addr fe80::1610:9fff:feee:e478>]
Thu Aug 24 17:29:27 2017 : sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Thu Aug 24 17:29:27 2017 : sent [IPV6CP ConfReq id=0x1 <addr fe80::1610:9fff:feee:e478>]
Thu Aug 24 17:29:30 2017 : Received protocol dictionaries
Thu Aug 24 17:29:30 2017 : sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Thu Aug 24 17:29:30 2017 : Received protocol dictionaries
Thu Aug 24 17:29:30 2017 : sent [IPV6CP ConfReq id=0x1 <addr fe80::1610:9fff:feee:e478>]
Thu Aug 24 17:29:33 2017 : sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Thu Aug 24 17:29:33 2017 : sent [IPV6CP ConfReq id=0x1 <addr fe80::1610:9fff:feee:e478>]
Thu Aug 24 17:29:36 2017 : sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>]
Thu Aug 24 17:29:36 2017 : sent [IPV6CP ConfReq id=0x1 <addr fe80::1610:9fff:feee:e478>]
Thu Aug 24 17:29:39 2017 : L2TP received CDN
Thu Aug 24 17:29:39 2017 : L2TP hangup
Thu Aug 24 17:29:39 2017 : Connection terminated.
Thu Aug 24 17:29:39 2017 : L2TP clearing por

Just to say that this was actually an IP issue - our DrayTek was not performing DHCP and this was the issue - I configured the DHCP relay under LAN > General Setup > DHCP Relay to no avail BUT under VPN and Remote Access > PPP General Setup > L2TP I configured the following:

See the full solution here:
https://blog.sircles.net/post/2017/08/25/iphone-ios-ipad-or-mac-osx-to-draytek-vigor-2860-or-3900-via-vpn-connection