The more the diagonal brace is screwed, the better, so you shouldn't screw it only at the ends. If you mean the door is crooked when you measure the diagonal, it can help to have a frame around the entire door. As your door is now, it lacks the long sides, as you only have the short sides, which seem to be in 45x70.L Larocc said:
So let's start from the beginning. First, screw together a square of 45x70 timber. The outer dimension should be about 3cm smaller than your door opening in width; the height dimension can be even smaller since the boards can hang over a bit as in your picture. Then double-check that both diagonals are equally long so it is perpendicular. When it is, measure in the diagonal brace so it sits centered in the corners, requiring cutting two angles at each end. This way, the diagonal brace rests a few centimeters on all sides of the frame and counters itself better. Then just screw on the boards. Place at least two screws in each board in the diagonal brace, and preferably some extra screws around the frame.
If you mean the door is warped because it doesn't lay flat on the surface, it's probably because today's timber is considerably worse than in the past. It twists and warps quite a bit if you got bad boards.
If you want to minimize the work to save your door, you can make another diagonal brace in the opposite direction, making a cross instead of building a frame.
As a reference, I have four doors in the summer cabin in the same design as you've built, and they are neither warped nor crooked. However, the doors are made with old timber and are probably 50-60 years old. They're slowly rotting away, but they're still perfectly straight.