A similar thing happened to me.
I used a DIY watercooling solution to cool my R9 290 cards (cooling was done in series through 5 cards). The radiator was placed outside of my mining room to my balcony. The idea itself worked great, my bedroom/mining room stayed cool and quiet.
But nothing can stay positive for an eternity
, the winter arrived. When the Claymore miner was open and crunching numbers, my GPUs put out enough heat to keep the water in pipes around 30C. But when the miner stopped for some random problem, that's when the s**t hit the fan. The temperature of the water inside the cooling system dropped to around 4C (often times even lower, the cooling solution was a mix between distilled water and antifreeze), cool enough to create condensation on the surface of my cooling setup inside my mining room. One riser took a deadly blow from this.
Woke up one morning to discover that 4 out of 5 GPUs were recognized by Win 10 and the 5th was not. I tried every different thing to get my miner working with 5 GPUs again but nope. The one GPU was not showing up. I kinda gave up and mined with 4 GPUs after this happening, thought that I had killed the one GPU. The mystery started un-tangling after some days when I randomly saw the riser (what connected the "faulty" GPU) again. One of the caps and the MOSFET were covered by this light blue crystal-like solid. What it was I still don't know, but it probably started growing after the riser got wet by the condensed water (one more riser was growing this weird thing on it, but it survived).
Have seen it a couple more times now, still have no idea what it exactly is.
https://ibb.co/cNg2dd my radiators outside on my balcony
