Gianni,
Ed B. has a good idea to isolate the divider from the mux. As you have soldering iron hot and ready to go, I suggest that you lift the resistors R1422 and R1100 at the -1.6V junction such that the 4051 mux (U1540) is not connected to the divider. Re-connect the resistors with a jumper wire and test the -1.6V point (low point of the divider).
If you need help to navigate the layout to determine which resistor ends to lift, I can post a layout map for the 11302A. I don't have the 11302 layout. Compared to the 11302, there were no significant changes in that area; so, the components should be in the same locations.
I am assuming that you have indeed verified that you have -1600V at the high end of the divider.
Clark
