Is it possible to work as an architect in the US with an Austrian degree in architecture (master degree)? Would it be possible or is it a must to pursue an "Architect Registration Examination" (ARE) with that background? Or would it be enough to be a licensed Austrian architect?
You may pursue licensure in the U.S. with a foreign degree but you must have your education reviewed through EESA - https://www.eesa-naab.org/
NAAB administers the EESA-NCARB program, which evaluates the credentials of foreign educated and broadly experienced architects against the NCARB Education Standard. EESA-NCARB provides assistance to individuals who do not have a professional degree in architecture from an NAAB-accredited school of architecture and who wish to either apply for NCARB certification or for registration by an NCARB member board.
Also reference the following information from NCARB -- http://www.ncarb.org/en/
You would NOT be able to practice architecture in the U.S. with your Austrian license.