Problem appears when adding mask

Hello.

Working with a project that involves a lot of invisible split screens, timeremapping, a bit of stop motion and text graphics. In other words: multiple layers and a lot of masking going on. A problem that is all new to me appears - and I am pretty sure that it have something to do with masks.

Using the crop-effect without mask works fine but when adding a mask - using crop or opacity or anything else I believe - a strange graphic behavior appears. Blinks, distortions, the mask turn all black, och all filled withs flickery colors. Some frames works fine, others don´t - all random. Sometime it places a black square or something -  in my image that don´t follows the shape of my mask at all.

When changing the mask, in a specific frame, the image changes everytime I move a keyframe - or during. totally randomly.

When adding a Adjustment Layer and adding any effect on it it all turns pitch black - even after rendering. If i nest the sequence and ad the effect to that it worked.

I´m working on a Macbook Pro 13 inch (early 2015), 2,7 GHz Intel Core i5, 8 GB 1867 MHz DDR3, Intel Iris Graphics 6100 1536 MB.

Using Premiere Pro 2017.

Please help!