A conveyor belt assembly (10) for transporting single sheets or stacks of a packaging material is described. It comprises at least one conveyor belt (12a) being arranged over at least two pulleys (20a-20d), wherein at least one of the pulleys (20a-20d) is adapted to drive the conveyor belt (12a) in a conveying direction (15). Furthermore, the conveyor belt assembly (10) comprises a conveying section (22) adapted for transporting the sheets or stacks and a return section (24) adapted to return the conveyor belt (12a) from an end (22b) of the conveying section (22) to a beginning (22a) of the conveying section (22). The conveying section (22) is equipped with a middle section (26), in which the conveyor belt (12a) is arranged in a substantially flat manner, and at least one transition section (28) arranged between the middle section (26) and the respective pulley (20a-20d), with the conveyor belt (12a) being arranged in the transition section (28) in a retracted position with respect to a plane (26a) defined by the middle section (26).