Underneath the white disk there is a microphone (a black round cylinder with a hole in it) that is activated when the alarm goes off.
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The white disk transmits the noise (caused by the tapping on the housing) to the microphone which snoozes the alarm.[br]
So it could be that either the microphone is defective or that is simply blocked by a speck of dust.[br]
I just blew into into it and it worked again.[br]
I do not know how easy it is to replace the microphone, as I have not yet take the lamp apart (it still has warranty)