The ending
One more who saw this film, yay :) Greetings from Slovenia.
I really liked the movie, I thought it was very clever, well acted and beautifully shot. What intrigues me the most is that David would in the end send 'the other' Maja and Leonie into a world where Leonie is supposed to be dead. I know they were running for their lives and any option is better than being dead but the consequences of that are only to be imagined.
Interestingly enough I didn't speculate on the door itself, it's obvious by how quickly David 'accidentally' kills off his past self that he is more than willing to give himself another chance for happiness and the origin of the door is irrelevant. For me, the door is an idea and a question, what would happen, what would you do, if you could travel a tunnel to your past and had the chance to correct your mistakes.