I hate to repeat myself, but: The scanner poles are not the cause for the UH-OH #123 errors here.
To make sure, I now totally removed the poles.
When starting the calibration, with or without clean data, the print head moves fast, then slows down in a position that seems to bee a bit higher than before, when auto-calibration still worked. It then moves further down with low speed, touches the build plate with a clearly audible click (that wasn’t there before) and, voila: #123.
I suspect electronics or software over mechanics.
Unfortunately, there is no workaround … or is there?