An update re this issue. First we (my son is a trained vehicle mechanic and is now studying engineering for vehicle technology and machine construction) examined the handlebars with the UJ. All are perfectly smooth-running and with good bearings. Then we turned the steering without linkage at the front wheel. It was very stiff and as before when driving we noticed this inequality of resistance.
We removed the steering because it was not ok to drive with it anyway. To see if the spindle or even the rack has damage to teeth the spindle was removed.
We noticed by our marking that the adjusting screw was much too tight. It was almost up to 1.5 mm outer circle of the screw at the stop.

We brought the parts to an expert and he confirmed that the spindle and the rack are still in good condition. Of course I was afraid it might be worn out, or a tooth might be broken off and ground.

But all parts are still very good. It was just too tightly adjusted. The rule of thumb with such an adjustment screw is about 1/8 turn back from the attachment point. There is also a spring under the screw so that the spindle always has pressure contact with the rack. Now the steering runs very well. It has been re-greased and is now being assembled.
I never noticed it before. Whether the rack can have curved over the years and kilometers in the 100th of a millimeter range so that the distance became somewhat narrower?

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'14 4/4 graphite grey