The seller is telling me that the products are original and new, and if they stopped working it is because of a too high temperature. But in my case, the temperature of the room is about 30-40°C and I don't think it should be an issue.
What are your thoughts about that ? Do you have any serious seller on alibaba where I could buy another batch of fans ?
The fact that the 20 out of 30 fans are working fine without breaking down probably indicates that the temperature is not a problem. Not considering that 30-40 degrees is already low, too high of a temperature would mean it's at the point where it could seriously harm your GPUs/ASICs as well.
I don't know any alibaba fan suppliers but i would strongly advise changing yours.