I have a 2007 CMax (1.6 petrol) from which sometimes, but only sometimes, the front seat passenger door cannot be opened from inside the car. There seems no particular pattern to this behaviour that I have been able to determine yet but it seems like this door alone is deadlocked even though I don't lock the vehicle when I am in it. Has anyone in this forum seen this before and if so, any idea what causes it?

Thanks,