This has always been a problem on mine.
It has two ways of sticking, firstly it is as if the rubber surround holds the door and secondly when you pull the handle nothing happens.
I have tried to cure the first problem by applying silicon spray to the rubbers, it helps a bit.
The second problem is just a matter of learning the knack of opening the door by 'snatching' the handle. Easier to demonstrate than explain, if you just pull the handle it will not open. At least it keeps the Mother-in- law out!
Thats my experiance in four years of owning a Ulysse.
Gordon