Excessive heat through a fuse holder is usually caused by poor connection/ short or excessive current draw.

My guess would be excessive current draw through the fan, the only other cause would be a short in the power wire that supplies the fan.

You can easily diagnose with the aid of a multimeter, set on ohms scale and with the plug at the fan disconnected you can check for short on the power side and if that’s ok, check the motor for high resistance, It should be less than 5 ohms, ideally 1-3 ohms.