The BM869s is actually pretty well sealed. There are gaskets/o-rings around the battery cover and the banana jacks, even the screw heads. So I don't think humidity would impact it.
But you could still fully inspect the PCB under a microscope looking for any solder issues or corrosion, clean selector switch and jack contacts.
A side note is that each jack's BeepJack feature does not function if their respective fuse is blown, so double-check those.
What 9V batteries are you using?