I have to be honest,
Had one trip also with a lot of rain and after 3 days it happened , BUT I did not found water neither humidity. Do not belive it is the reason.
What is damagged on the unit is a component that handle all the current and that's why the heat sink... Probably it really needs cooling and can not be sealed.
I am just affraid what to do to avoid that situation next time. I am going to replace the module with the SAME revision, so what changes and why it should not happen again....
A spare could be good solution but really silly and wrong way..