No one answered this really simple question. 
Xenon was closest with his “leaves lice” (=greenfly) but it was not correct. That leaves EUR 20,- in my pocket. I just have to think about how to squander it. 
It is a “dust lice” (col.) or Psocoptera (explanation: English, German), probably Dorypteryx domestica. They are often found in new buildings (because of the remaining humidity in the concrete). The Psocoptera are absolutely harmless as professors at the University of Natural Resources and Applied Life Sciences, University of Vienna and University of Innsbruck told me. I sent them a macro picture and asked for a classification. It’s great to have some academics at hand when you need them. 
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