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Thanks guys, for all the answers.
I disconnected the external buttons, and now the U3S starts up with "Diagnostic".
After pressing the button on the PCB, the splash screen appears, and I can further configure the device.
So, it does seem that the pin 25 button was shorted to ground. However, measuring the button without it
being connected to the PCB there is no shortcircuit (only when I press the button, of course......).
So, I'm still not sure why this happens.
Will investigate further. For now, the U3S is usable again. I just can't configure it using the external buttons...