Now, a final system test was finished using the display to show the result:

Here you see the result in the accu:

And here the result using the display...yes, the first character is not part of the message, problem here is a missing synchronization between the arduino controlling the display and the relay system. And, problem number two,  the system test software is still using the address of the I/O register (=2), this could confuse the I/O decoder and the arduino. But I don't wanted to change the system test software, that would lead to changing a lot of memory addresses of the software.