Thank you!
The problem is, that the sequences were already nested, however I tried to render each sequence and then imported them into a new project. Still the same problem without any of the layers having any effect or else applied.
But now I noticed a really strange thing. When I have “Motion” selected in the mask layers Video Effects everything shows up fine in the preview window as long as motion is still selected. if unchecked or changed to another layer, the right result disappears again….don´t know what to do, I use two layers and want to have one mask applied to one of the layers, have one footage on layer one, the second on layer two, the mask on layer 3 and a track matte key applied to layer two so it uncovers areas of layer 1 … can´t figure out what is wrong, other ideas?