I vote for a foreign object. Look for something stuck in the lip of the door or on the mating part of the loader body - in your last picture it looks like there is something amiss at the right side of the loader body lip. Also look for something in the door pivot area - a bit of broken off film maybe.
The loader looks well used so it worked properly at one time. Whatever it is, it isn't a manufacturing defect. Bakelite doesn't warp with time, so that can't be the cause.