Something odd is happening if connecting the PanelDue causes the VIN reading reported by DWC to drop. My best guess is that in the cable connecting PanelDue to the Duet, the Dout pin is shorted to the +5V pin. Or perhaps you are connecting the PanelDue cable to the wrong connector on the Duet?
Due to the risk of damaging the Duet, I recommend that you do not connect the PanelDue until you have resolved that.
I found the problem. It was the two jumpers which were on the second z step. After I removed them the web shows 24V and the PanelDue is also pluged in. Maybe there is some issue in the circuit. But now everything seems to be fine. fingers crossed
Edit: Not the two jumpers are the issue. It's the 10pin cable from the PanelDue if I connect it to the Duet 2 then the voltage drops. The other 4 pin cable have no effect on the voltage.
Edit 2: Oh man shame on me... I connected the 10pin cable to the conn_lcd because of his labeling but it should connect to the conn_sd. I'm so sorry.