A service error occurs when you try to perform any export operation.
When the cube property is explicitly defined in the connection string of the data source and one of the characters' case is not correct the export will fail. E.g. you have a cube Controlling but you specified Cube=controlling
Please change the connectionstring in your report to Cube=Controlling (instead of Cube=controlling). The cube is named controlling with an uppercase C within Analysis Services while you refer to controlling with lower case c. Even though DWEX Designer can cope with that the export can't.