I found the problem (and solution).
The user did create some sort of graph with calculations behind it which had problems to load.
This graph was behind an existing graph. Only the user who created this object can see it in the report ...
This explains why only this certain user had these problems by opening the report ...
Simply removed the object (when we could open the report accidentally) and the problem was solved.
Does anyone know a way of removing the settings for a user without having to login as the user itself??