I took the rear right joint apart and measured the different diameters:
The inside diameter of the plastic insert 100-07 is OK, but I can see that the holes in the lower tube are slightly oval. Which I don’t understand, because that would suggest that the plastic insert is continuously sliding in the tube, which is definitely not the case as it was hard to get it out of the tube.
I also stated that the cup spring continued its abrasive work on the lower tube, even after I had rounded the sharp edges and put some grease some time ago. After a while there will not be enough tube material anymore 
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I think I can get some spring steel shims at my job, that I will put between the tube and the cup spring. This should definitely reduce the wear.
BSA, could you please check the hole diameter of the lower tube of your Strida when you have the chance?