I know my answer might not be completely related to the original post since I didn't have the problem of the fan not reacting, it only didn't want to go to the OFF state.
It did change to LOW from time to time, but most of the time it was on HIGH.
I "fixed" the problem of the noisy fan by taking off the top of the unit and putting there an 80mm silent (Noctua) fan connected to the USB port of the NAS. The fan blows air downwards over the HDDs. Now the unit always reports as being around 40 centigrades (103 Fahrenheit) and the HDDs report as having 27 centigrades (80 Fahrenheit).
Because of this, the internal fan is OFF all the time.
I know it's not the most elegant solution, but it works great. Now, all i have to find out is how to put a filter between the fan and the unit so it won't get filled with dust.
P.S. the fan just "sits" on top of the HDDs for now, I'll have to see if i can make a support for it. I don't exactly want to modify the top of the unit as i would have to drill a big hole in it for the air to enter
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