Well I have an update: The problem was not related to the vera firmware but to the size of my network. I just tested the inclusion process on a test vera plus with 0 devices and the latest beta firmware and sure enough it exchanged the security key without any problem. @Agnes.Zooz, considering this result, I believe that it is purely a matter of timing. I think that with a large network, the vera is not queuing zwave commands like it should and potentially could either drop the secure class exchange command or delay it significantly to the point where the sensor has already timed out. If it would be possible to extend the time out during which the zse40 sensor accepts to exchange key during the inclusion process, I would happily test it. This would also explain the luck factor where I occasionally managed to get it included properly.