unfortunately those NSM models are SUPER complicated. it's German over-engineering at it's best. the wiring schematics are even in German with some engaleese sprinkled throughout. not too bad once you figure out the colors in German...
red — rot
orange — orange
yellow — gelb
green — grün
blue — blau
purple — lila
gray — grau
brown — braun
beige — beige
white — weiß
black — schwarz
the main power enters the amp board in the long plug with the multi colors on the left side at the back of the case (behind the CPU board) there should be a row of fuses there) check them all. while you are at it, unplug that connector and plug it back in again a few times....it can get corrosion on it and cycling the connector off and on again cleans it off.
check the plugs going into the CD unit...this is what the machine is complaining about. there is a circuit board inside the CD player cabinet...that is the decoder board. check it's plugs too. something may have come loose.
it's not out of the realm of possibility you have low power at your socket. check it with a volt meter.
on some of those machines, if you where to pull the bottom metal plate out of the bottom of the cabinet, you'd see several "taps" for the power transformer. you'd set it to the closest voltage you have at your wall. I personally have never had to mess with it, but someone may have messed with yours previously if the voltage it was at was high. it could cause your system voltage to be low or marginal. it's can't hurt to CHECK it. (no touchy though)
standard disclaimer applies, setting the voltage is hazardous and somewhat complicated, and best left unless you have the manual that explains the procedure...as incorrectly setting it can cause nasty things to happen including but not limited to killing you and/or burning your house down.