L2TP/IPsec - Android to FVS336Gv3 - "the length of the isakmp header is too big"

Here is an iperf from my Win10 PC (172.16.16.10) to the phone (192.168.251.14); the phone is a Galaxy S4 running nothing but the "iperf for Android" app shown here https://play.google.com/store/apps/details?id=com.magicandroidapps.iperf . Bandwidth starts decently fast for around 10 seconds, but then there is a 10 second period of no traffic, followed by decent bandwidth, followed by a disconnect in the iperf program. Immediately after the disconnect, there was another ~10 second period of no traffic to/from the phone. I can re-run this test without the VPN connected and I get consistently high bandwidth with no periods of loss.

Here are the logs you requested. IP addresses have been blanked. As I refresh the logs, this "isakmp" message appears roughly once every 10-30 seconds:

Is there an official recommended way of connecting an Android phone to an FVS336G via IPsec? If so, I haven't been able to find one, and this is the only gateway I've ever used that had so much trouble with the built-in Android VPN client. As an aside, I tried using a different IPsec VPN client on Android with an app called "NCP", as some other posters have said good things about it. I have identitcal problems with that VPN app as well.