Hi @Thomas2,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Ultimately, the best way to repair this issue is to remove the angle and start again. If you'd like to do this, have a look at How To Attach And Plaster An External Angle. This handy guide will help you through the process.
I can offer a solution that won't require removing the angle and starting again, but I can't guarantee it will be as seamless as it was.
The cracking has occurred because the external angle has shifted when you slammed the door. The first step is to lock the angle back in place, which you have already done with the staples, you just need to make sure they are going through the small holes in the metal angle. You will know when they aren't because they will hit it and bend.
Once the angle is secured in place, you'll want to cut a V-shaped groove in the plaster, essentially widening the crack. This is to ensure all the loose material has been removed.
You would then use Builders Edge 50mm x 20m Fibreglass Builders Joint Tape along the length of the crack to give the repair some strength.
You'll then apply an all-purpose compound such as Gyprock CSR 15kg Easy Flow All Purpose Compound, sand it smooth with a sanding sponge and then paint.
Let me know what you think and if you have any further questions please don't hesitate to ask.
Jacob
