Have a look at the front end of the (outside) waist level track. You may need to remove the cover panel to see clearly.
There should be a plastic 'buffer' piece in the end of the track where the top rollers rotate inwards to close the back of the door onto the latch. If this is missing, the mid level roller/latch will move too far forward, taking the door with it.
E D I T - in your 4th photo, between the door seal and the cover panel (at the level where the hedge forms a green background) you can just see the slot for the buffer. Can't tell whether or not it is empty, though.
This buffer is a 'T' section, laid on it's side with the stem popped out through a horizontal slot in the end of the track. (251 843 876 limit stop)
http://www.vwbusshop.de/251843876-Limit ... ng-door-T3" onclick="window.open(this.href);return false;