Why VPN concern then? Well My general thought is to route all my mobile traffic over my VPN connection when I am not home. I have this working, with a Androind Tasker task that turns on VPN client in my mobile when I leave my WiFi connection (that works just perfect by the way). So I end up with maybe 8-12h VPN connection active per day. Could this create this error when it hangs? I will also connect my wifes mobile like this, and most likly my parents router also and so on, if I can trust the VPN connection... (well if I end up with that much VPN usage I guess VPN port knocking is a bit unnecessary).
There is a confirmed memory leak issue in 380.58. Serious or not depends on people's usage scenario. Details here: http://www.snbforums.com/threads/sbin-preinit-memory-leak-in-380-series-fw.31622/
Perhaps your VPN usage scenario is hammering the leak in /sbin/preinit the hard way. Check the memory usage as outlined in the above thread at 12h, 24h, 36h, 48h points.
If you see an monotonically increasing VmRSS, then you hit the jackpot..