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First of all to be clear, the camera was not dropped or anything; it started to behave like this from nothing.
I turned on my camera and first it displayed "Turn off the camera and turn on again". After that, it constantly showed the "Press shutter release button again". Normally I first searched Google for the problem, and all the threads I could find was shutter mechanism needs to be lubricated, or the mirror was jammed etc. All these were due to dropping the camera.
I took the camera to Nikon, and they asked ~250 EUR to fix it. Since I don't have the money right now, I got it back home and turned it on - tada! Camera is working again. I took about 50 pictures when it showed the error again. This started to happen every time: I leave it for couple of hours, turn it on, and take ~50 photos before the error shows again.
I check the mirror, shutter blades and everything, but nothing looks stuck or jammed to me. I'm really puzzled by the "cooldown" time it needs to be working again. If the shutter / mirror / blades / whatever is jammed, it won't work at all.
Does anyone else had this problem? How did you resolve it?