After I had no idea in 2018 (see tip at the time), I researched and found out that it was the Skaby Castle. This has an eventful history:
Monks stung peat there until 1915, farmers planted until 1939, around 1943 the castle was used for the "Lebensborn" campaign, around 1947 it was a recreational facility for mentally ill Russian officers and until 1989 it served as accommodation for the Army Sports Association (ASV) , the Stasi as a place of recreation and the leadership of the former GDR as a place for private parties. During the GDR era, a 3 meter high concrete wall was built around the entire palace grounds using building elements from the "Berlin Wall". After 1990, the palace grounds served several times for various festivals.