You don't use your setters. At Bruch b1 = new Bruch(zähler1, nenner1);
you set your fields in the constructor, not the properties.
Change your constructor to set the properties:
public Bruch(int zähler, int nenner)
{
Zähler = zähler;
Nenner = nenner;
}
You could've caught this by putting a breakpoint at the setters and see they're never called, then track back through the code to where the values are set.