Of the family Aradidae, 30 species out of 57 European species are represented in Austria. (Source: Fauna Europaea)
They commonly live under the bark of dead trees, many of them feeding on fungi, this is called being mycophag.
Their extremely long proboscis, often several times longer than their body, is in rest position rolled up inside the extended head capsule.
Worldwide there are more than 2000 species in over 200 generas.
Read more in Wikipedia.
Some species of the Aradus, like many other insects, are very hard to identify. Like this one - it could be one of 3 or 4 species. My guess is Aradus cf. betulae.