NetworkTransport disconnect every 40 seconds

Hi,

I'm using the NetworkTransport API for a turn-based game, in pretty much the same way as this tutorial explains: http://www.robotmonkeybrain.com/good-enough-guide-to-unitys-unet-transport-layer-llapi/
I have only two players, they successfully connect to each other by IP (not matchmaking), and successfully exchange messages every second or so.

However, every 40-50 seconds, the clients receive "Disconnect" messages, with the error number 6, aka "Timeout". The disconnects...

NetworkTransport disconnect every 40 seconds