The service manuel is very helpful, we where able to restore the device to its working operation again.
Text excerpt from page 88 (click to view)
Chapter 7: Accessories The paper tray of the cut sheet feeder has a maximum capacity of 100 sheets (90 g/m2) or 170 sheets (60 g/m2) of standard paper. A mark on the left guide rail indicates the maximum amount of paper. Read also the further notes on paper in Chapter 3. 1. Set the paper set lever at the right end of the cut sheet feeder upwards to the RESET position. The paper tray will open towards the back.
Direct pressure on the platen can cause damage to the print head and pins. Make sure the paper guide is correctly adjusted. 2. Take a stack of paper, fan it to part the sheets and insert it into the sheet feeder. Make sure the stack of paper is lying on the bottom of the tray. 3. Make sure the rear paper stands are positioned equally between the paper guides.