The fix is to tighten the strap - there are two ways

1 - Remove the frame with wheels and rollers and re-align + tighten the strap

2 - Tighten and partly re-align the strap

I opted for solution 2

I tightened the strap at the botton of the frame, because this is where I had space to manouvre

I positioned the strap more to the left so it got centered

I used a 1.5 mm drill to prepare a whole in the frame so the plate screw (See picture 2) does not rapture the woodden frame

I used a plate screw to secure the strap in the new position