Well, that's going to be a BIG problem.
Mass
The largest creature that has ever flown was about 250 kg. Now, while your creature is double its weight, a 250 kg pterosaur still had people questioning the limitations of biological flight.
Indeed, burst-flight performance seems not to intrinsically degrade with increasing size.
So, let's talk wings
While aerodynamically efficient, an albatross's monstrous wings would have tremendous inertial costs when it comes to flapping during take-off. Keep in mind that flapping is an oscillatory motion, unlike the rotation of helicopter blades, or the forward motion of a fixed-wing aircraft.
Thanks to how rotation works, a wing with a length of 5 meters can achieve the same speed at a much lower angular velocity, compared to a shorter wing. Large fliers thus tend to have lower flapping amplitudes.
What you must keep high is the flapping frequency, i.e: how many times does the dragon beat its wings in a given unit of time.
So, high frequency, low-amplitude, this is where another important element comes up:
Square-Cube Law
Muscles scale allometrically, they get extra thick but remain short, which comes at the expense of the range of motion. There's a fixed value of absolute shortening the flight muscles have to achieve to move the wing around, depending on the flapping amplitude, once that's met, they can be as thick as necessary.
There's even more to it, but I'm too tired to add it in right now.