PCB outdoor moisture effect and protection
I have done extensive tests and eliminated all the above hypotheses.

You are very sure of yourself, especially for someone that isn't sure what they are doing.  Good troubleshooting includes keeping open the possibility that you screwed up.  From your report of what might have gone wrong the first three times, this is a definite possibility.  From your own description, it's pretty clear you don't have the expertise to decide that the three other problems have been "eliminated".  That's the kind of arrogance that blinds you from seeing the real problem.  Debugging must always be done with a humble attitude.

It is possible that environmental conditions caused failure.  Of course that depends on what the environmental condition were.  You mention cold as a possibility.  How cold?  How was the board protected?  What temperature range was it subjected to?  Could moisture have condensed onto the board?  Could bugs have crawled in?

You mention both cold and battery as causes of failure you considered, but apparently independently.  Batteries are more susceptible to low temperatures than most other components.  The battery may work fine powering your circuit on your bench, and the circuit may work fine in a cold chamber while powered from a external source.  However, put the two together and it may not work so well anymore.

A more detailed answer requires a more detailed question.