You don't say what year Doblo you have, so I can only refer to my later model.
The hook on the lower arm should be free to move.
At the end of the cable on the latching mechanism, there is another small hook - this latches on to the larger hook to hold the door open. Make sure the smaller hook is free and that the cable hasn't seized (probably the most likely cause).
It's all easily accessible.
It is worth investing in some aerosol grease and keeping the mechanism liberally coated. Likewise, the tracks for the door slide are exposed to the elements, and the lower slide is notorious for filling up with road cr*p, particularly in the winter, and it needs to be lubricated regularly, otherwise your door will slowly get stiffer and eventually refuse to move (at the most inopportune moment!)
Mike