Thanks for your help and the link. I already know the documents onbitsavers.
Unfortunately there's only a short description of this error, but not how to try to fix.
We checked the voltages, so it seems, that power supply is OK. (We haven't tried to check, if it's stable enogh --> old capacities...)
But how can we find a problem on the CPU board?
Or is it uncommon or not possibile to repair a CPU board instead of replacing?