Nachthelle (2014) - IMDb 
Two couples visit a home from their past, and sexual tension brings out the worst in each other. Long-buried grudges resurface.
Director: Florian Gottschick
Writers: Florian Gottschick (screenplay), Carsten Happe (screenplay) | 1 more credit »
Stars: Anna Grisebach, Vladimir Burlakov, Benno Fürmann | See full cast & crew »

Storyline
A swingers tale: A relaxing weekend in the country doesn't go quite as planned for two German couples. Anna, around 40, and her much younger boy toy, Stefan, have come to the former's quaint childhood home, tucked away in a lush, green forest in former East Germany. An old friend of Anna's, Bernd, is already at the cottage with his boyfriend of many years, Marc. The two couples are all set to enjoy the beautiful surroundings and each other's company over a long summer weekend. But like the open-pit mine activities that are happening nearby and sending unexpected small quakes throughout the region, there are things in Anna's and Bernd's past that start to influence the present, beginning with the revelation that Anna and Bernd used to be not only classmates but also lovers.

www.imdb.com/title/tt3404246/