Unable to connect to VPN (L2TP / IPSec PSK)

[GrilledPizza]

I'm unable to establish a connection to my VPN, type L2TP/IPSec PSK using an IPSec pre-shared key (local and external). The VPN is running on a Synology NAS (model DS713+ with DSM 5.1-5022 Update 2).

After configuring it hangs on "Connecting..." and after a while it says "Disconnected". When I look at my VPN server logs, nothing seems to be happening on the server as if nothing tried to connect.

My old LG Optimus 2X running android 2.3.4 is able to connect using exactly the same settings and within the same environment, which makes me quite sure about the Xperia Z1 Compact being the culprit.

Software

14.4.A.0.157

Android

4.4.4

Model

D5503 (unbranded)

Is there a way to debug this on the Z1 Compact? Or is this an known issue? If so, is it going to get fixed?

//EDIT:

Apparently I'm not the only one with this issue on Xperia devices:

1. http://talk.sonymobile.com/t5/Xperia-Z-ZL-ZR/VPN-Xperia-Z-don-t-connect-S3-mini-does/m-p/935450/high...
2. /t5/Xperia-Z3-Compact/cannot-make-a-l2tp-vpn-connection/td-p/936805
3. /t5/Xperia-Z2/VPN-L2TP-not-working-Android-4-4-3/td-p/843177
4. http://forum.xda-developers.com/xperia-z/help/l2tp-vpn-connection-fails-t2865147
   

[jait]

H Rickard, I have a z5. my Vpn still doesn't work. My BLE still doesn't connect to certain wearable devices. Any updates since this last post? 

[Rickard]

@jait

There's no update available for Z5 yet fot this i'm afraid.

If you are unable to connect to certain BLE devices, please make sure that you have downloaded and installed to correct host app for those devices.

[JAIEMS]

Already running on Marshmallow. Seems this issue with L2TP/IPsec PSK has yet to be resolved.

[JAIEMS]

Got a z5 dual

[toits_]

Sony Z3 compact still has this problem, L2TP will not connect no matter how. Software updated to the latest as of 03.2017

[Ghazi]

Very frustrating. Running Android 7 on my Z5.

My company just installed a new Draytek router and i cannot establish a VPN connection. The built-in android VPN, and the Draytek Smart VPN app simply will not connect. Had tech guys spending hours of time to resolve this. Then they realised that this problem is unique to Experia. Other android phones are ok.

Come on Sony. I love my Z5 but this is a critical problem for me. I am going to have to ditch your phone. I have had Sony phones forever, but this breaks it for me. I cannot work without​ being connected to the company VPN.

[Ahmedelbadry]

I have Xperia Z+  and unable to setup ipsec xautn Psk VPN using built-in Vpm app as it disconnect immediately,  is there any solution to overcome this issue 

Build number 

32.3.A.2.33

Android version: 7