LEDE 2685 crash/reboot on Kirkwood Viper if WAN is connected

Hi all
I have just compiled the rev 2685 for Linksys Viper, which I run on an EA4200v1
There is a quite big problem: if I have the WAN connected to my ISP router, LEDE reboot after few seconds after the startup. It's so fast that I'm not able to retrive any usefull logs/dmesg
It's enough to disconnect the WAN for having the system up and running fine
I have compiled into the image the following packages:

luci openvpn-openssl miniupnpd tayga iperf3 6in4 6to4 6rd kmod-gpio-button-hotplug kmod-ipt-nat6 luci-app-openvpn luci-app-upnp luci-app-uhttpd wget adblock luci-app-adblock lm-sensors nano