Thank you all for your comments and hints. I guess my test equipment is not perfect for these tests and now I am not sure that the platen itself is the problem.
I measured the voltages going into the comparator/op-amp U102.
a: UREF 5V is actually 5.21V in my case (measured at right side of R119)
b: behind R119, at (+) of U102 I see 5.04V (measured at left side of R119)
c: between R109 and R110, at (-) of U102 I see 5.20 V (measured at left side of R109)
(right and left as seen from the rear of the open plotter)
My plan is now to first correct UREF to be closer to 5V. In fact this may be my fault as I replaced the broken 4.99V Zener VR12 on the main PSU board by a 5.1V Zener because I did not have a 4.99V at hand. I believed that with the tolerances this might be o.k. but just believing is only an alternative fact ... So I'll order some 4.9 and 5.0 V Zeners to see what happens .next.