What does this placard mean on some Vienna's buildings?

What does this placard mean on some Vienna's buildings? - Blank white placard hanging on wall covered with white tiles inside of building

I've seen this on several buildings in Vienna in Innerstadt, a few weeks ago. Wondered what exactly it meant.

vienna

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What does this placard mean on some Vienna's buildings? - Vintage posters and on shabby rusty wall

What does this placard mean on some Vienna's buildings? - Placard of a February 14

What does this placard mean on some Vienna's buildings? - Free stock photo of activism, adult, advertising

Billy Joel - Vienna (Audio)

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