When I powered up my printer I saw that the Y axis loses a few steps. the resistance in the edge positions was too high. I did not recognize that when I moved the printing head by hand during assembly. For me it was OK. But as I took a look at the rubber belts I saw that one of them was slightly moving to the bottom when I moved the head to one of the end positions. The movement of the belt was clearly visible while I could not see any movement of one of the rods. But it was clear what was happening. The rods were bend by the printer head. Not much but enough that the belt lost a bit of its tension. So I fixed the distance between the printer head linear bearing clamps. (Just a few 10th of a mm). After a bit of fiddling I found the perfekt distance. It now works like a charm.
Happy to read you were able to fix the problem.