The service manuel is very helpful, we where able to restore the device to its working operation again.
Text excerpt from page 7 (click to view)
DISASSEMBLY INSTRUCTIONS
1. Remove machine screws M4.0 x 8.0 (1 to 6) from the side panels. Remove tapping screw 3.0 x 8.0 (7 to 10) from the back panel. Refer to Figure No. 1.
Figure No. 1
2.
Pull both sides of the TOP COVER slightly outwards 11 and tilt approx. 35 ° and then remove in the direction as indicated by the arrow 12. Refer to Figure No. 2.