Hi, Marcus,
I am not able to duplicate your observations. I am running iMovie 10.1.10 on Mojave and the crop tool works as designed. The function of the two intersecting yellow lines is to show the vertical and horizontal midpoints. You move the white corner markers by placing your cursor on the marker and dragging while pressing down. You reposition the adjustment rectangles by clicking in the body of the rectangle and then putting your cursor on the edge of the rectangle and dragging while pressing down.
It might help if you would post a step-by-step of what you are doing to utilize the crop tool.
To cover the bases, here is a link to the on-line help menu that explains how to use the crop tool:
https://help.apple.com/imovie/mac/10.1/#/mova56757e7e
If still not working, try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
-- Rich