I think I am on the right track why the Panel does not work...
This is Table 3.4 from the data sheet
If you look at pin 26 and 27 and both columns you will notice contradicting names...
And Murphy did hit us: I did take the first column to connect -5V to Pin 26 and +5V to Pin 27.
Which appears to be wrong (fortunately there is the schematics of the small pcb already attached to to the display and I could remove some isolation foil and directly measure where C6 and C8 are connected to).
So this explains why a connected display makes the +/-5V break down (it is fortunately current limited so that it is unlikely that the panel is damaged).
BR, Nikolaus