Are your R/O filter cartridges up to date? If the water quality is not the issue and the soil never totally dries and no sudden humidity drops I would say a toxic soil condition probably occured. Perhaps it became too compacted and created noxious conditions. Do you use peat? My neps die in peat because of my conditions and watering schedule but they do wonderful is LFS and fine bark chips. I don't believe in the "mystery death" syndrome it happened for a reason you have to identify why so you can avoid it from ever happening again.