However, now I am again experiencing again the first symptoms as half a year ago: The cooling fan is unusually loud (i.e. turned on all the time) and the laptop feels hotter than normal. Therefore, I assume that there is again too much dust inside.

Please note that I use the laptop ONLY on my desk in an office environment! Is it normal that dust causes such problems? I don’t think exchanging the motherboard every six months is a solution :D But what is? I certainly have no clue how to open the laptop by myself.