World won't load after riding a donkey/player lost

I closed Minecraft while riding a donkey and when I tried loading the world I was stuck on it, couldn't move, neither use any keys. Moreover the world was completely blanc and not genereating anything.
I tried moving the player in McEdit and repairing the world, but I got an error saying "u player not found".

Somehow I am now able to load the world with one chunk generating, but I can't playce any blocks or do anything but walking and the game will freeze after a while. This just happened to this map.