failed to connect to the VPN server

[Grag]

1. VPN server operating system: CentOS 6 x32, bandwagonhost virtual private server.
client operating system: iOS 8.4 or 9.0 and OS X 10.10.4 or 10.11 with built-in L2TP.
2. result of ifconfig:
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:34 errors:0 dropped:0 overruns:0 frame:0
TX packets:34 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1696 (1.6 KiB) TX bytes:1696 (1.6 KiB)

venet0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:127.0.0.1 P-t-P:127.0.0.1 Bcast:0.0.0.0 Mask:255.255.255.255
UP BROADCAST POINTOPOINT RUNNING NOARP MTU:1500 Metric:1
RX packets:10890830 errors:0 dropped:0 overruns:0 frame:0
TX packets:9198908 errors:0 dropped:12575 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:8140301868 (7.5 GiB) TX bytes:8265117702 (7.6 GiB)

venet0:0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:45.62.99.69 P-t-P:45.62.99.69 Bcast:45.62.99.69 Mask:255.255.255.255
UP BROADCAST POINTOPOINT RUNNING NOARP MTU:1500 Metric:1
3.result of uname: Linux
4.The build number of SoftEther VPN: Ver 4.18, Build 9570, rtm
5.I only used the SoftEther VPN server installed on the CentOS vps, and connect to it on iPhone with built-in L2TP
6.I'm not sure if there is a NAT or Firewall between the virtual private server and the internet.
7.Yes, I'm using SecureNAT. It seems I can't use Local Bridge because the server is a virtual machine and does not have a physical network adapter according to the Server Manager tool's warning.

The problem is that I actually can connect to the server one or two times a day, but most of the time can't.
the error message shown on iPhone or Mac is:
The L2TP-VPN server did not respond. Try reconnecting. If the Problem continues, verify your settings and contact your Administrator.

Below is the log of a successful connect and a failed conncet
successful connect log:
015-08-26 00:09:46.294 IPsec Client 178 (183.62.57.244:6277 -> 45.62.99.69:500): A new IPsec client is created.
2015-08-26 00:09:46.294 IPsec IKE Session (IKE SA) 91 (Client: 178) (183.62.57.244:6277 -> 45.62.99.69:500): A new IKE SA (Main Mode) is created. Initiator Cookie: 0x49F31B1C3DD2FAF5, Responder Cookie: 0xDE4140D3656916FA, DH Group: MODP 1024 (Group 2), Hash Algorithm: SHA-1, Cipher Algorithm: AES-CBC, Cipher Key Size: 256 bits, Lifetime: 4294967295 Kbytes or 3600 seconds
2015-08-26 00:09:46.781 IPsec Client 178 (183.62.57.244:6291 -> 45.62.99.69:4500): The port number information of this client is updated.
2015-08-26 00:09:46.781 IPsec Client 178 (183.62.57.244:6291 -> 45.62.99.69:4500):
2015-08-26 00:09:46.781 IPsec IKE Session (IKE SA) 91 (Client: 178) (183.62.57.244:6291 -> 45.62.99.69:4500): This IKE SA is established between the server and the client.

failed connect log:
2015-08-25 23:47:22.441 IPsec Client 177 (183.62.57.244:39793 -> 45.62.99.69:500): A new IPsec client is created.
2015-08-25 23:47:22.441 IPsec IKE Session (IKE SA) 90 (Client: 177) (183.62.57.244:39793 -> 45.62.99.69:500): A new IKE SA (Main Mode) is created. Initiator Cookie: 0x93F197C0A0AADF31, Responder Cookie: 0x78455556E2B7359A, DH Group: MODP 1024 (Group 2), Hash Algorithm: SHA-1, Cipher Algorithm: AES-CBC, Cipher Key Size: 256 bits, Lifetime: 4294967295 Kbytes or 3600 seconds
2015-08-25 23:47:32.444 IPsec IKE Session (IKE SA) 90 (Client: 177) (183.62.57.244:39793 -> 45.62.99.69:500): This IKE SA is deleted.
2015-08-25 23:47:32.444 IPsec Client 177 (183.62.57.244:39793 -> 45.62.99.69:500): This IPsec Client is deleted.

[thisjun]

Could you try using OpenVPN?