After mounting the new command unit into the control board, I tried again to add all numbers from 1 to 100. And...the system failed. The result was wrong. So I tried a lot of things, the last thing I tried was to simply load 0x5555 into the accu register. And the result of the accu was this:

So...two lines of the databus are connected. I disconnected some cables and now I am sure that in the marked area of the address calculation board is a connection between databus[3] and databus[4]. This wasn't a problem until now because in all tests in the past I did not fully connect the databus to the address calculation board.

The unfunny part is that the board is connected with the rest of the system and I have to disconnect it...