Malyan M180 Y--axis belt tensioner

When printing with my M180 I found that the y-axis wasn't steady, this could be seen on the test cube 20x20 and when printing a round wall of only 2 extrusions. In the cube case the corners were not correct a notch was on 2 opposite corners. in the round wall case one could see a separation of the extrusions on 2 opposite locations, this was where the y-axis changed direction. When the printer does extrusion clockwise and counterclockwise this is very clear.
The problem that causes this is not