@FINAO,

Have you verified that the device is not being put under stress or overloaded?

The fact that it fails after that long of a period, indicates to me that something is being done to the switch that is either causing the failure or overloading it to such an extent that the damage is being done gradually

  @FINAO,

Definitely check this. There are cases where fixtures require more power than usual, or you may have less than efficient light bulbs.

While the switches were functioning, did you notice anything different about them? Such as the light not being as bright, a humming noise, or problems dimming at low or high brightness? 

You may want to check to make sure all the connections are secure and that there is no other device running alongside the wires that could be interfering or causing a short. I would start by checking to make sure that the other switches in the box have secure connections.

Lastly, I believe there is an overall limit to the length of wire over which the units can communicate with the light reliably due to the resistance experienced over the longer line.

Are your switches further apart than your others, or is it possible they have a longer run?