Thanks a lot for the hints.
Today I resoldered the 2 74HC595 today but there is no difference. The orientation of the resistor networks is correct andI think they are in line to read encoders and buttons which is working fine.
There is a resistance of 43 kOhm between the QA-QH outputs and the ground connector of the 595 shiftregister, is this normal or shall I look look for shorts?
I can also measure a resistance between the outputs of the shifregister 1 and 2 (Out vs DOUT) but these are serveral MOhm, so I don't think this is the root cause of the problem.
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Is there a good methology to track down the root cause?