During the disassembly of the board I somehow "delaminated" the display. The result was that the display (after the battery was changed and the device closed again) looked "cloudy". From some angle you couldn't see anything at all anymore. The solution was to heat up the hole device with a hair dryer and rest it on it's front side for about an hour. After that it looked absolutely perfect again. I am not sure whether the heating is really necessary (maybe just lying the ipod on the front side over night would have the same effect) and surely one should be careful to not overheat the device.