What killed it? Arduino in resin
Potting of circuits can lead to various issues. It can't be told exactly what caused your device to fail until you uncover it like an archeologist.

Common problems are:


Resin entering a spring loaded contact and canceling out contact forces
Resin exsuding corrosive substances ruining contacts
Resin expanding during curing and opening spring contacts or even breaking up solder contacts
Same effect after cool down of resin


Later and often unexpected problems are:


thin gaps around potted objects (i.e. no surface contact) literally sucking in water. 
potting material producing water itself during curing.


Many of these problems can be circumvented by applying conformal coating, as suggested by comments and apparently proven right by your experiments. 
If it is not necessary refrain from using sockets and solder connections directly to the components. You won't be able to salvage them, anyway.