played around with resizing to an other aspect ratio in Tools and surprisingly now there not only one/1 green “box” but even three/3 and I have the impression that resizing to the desired AR does not work anymore properly.
Julio’s right – the solid green boxes are only guides for safe title / safe action. If they are confusing, you can disable them in the grid menu on the right.
The dotted box is your adjustable reframe. It appears that the 1.5 aspect ratio is set wrong if you want maintain the original HD aspect ratio. To change it, I’d go to ratio and set it to 1.778. Keep the scale set to proportional to maintain the AR.
You may also want to change resizing ‘letterbox’ to ‘crop edges’ if you want to lose the pillarbox.