While knowing you said you tensioned the short belts, I am still inclined to think the problem lies with your short belts. Do you have a photo of some reference on how tight they are? Perhaps pushing against it with your finger?
And can you move your printhead around with just 1 finger pushing against your print head? How freely can it move / how much resistance do you feel?
I don`t have a Ultimaker, but had the same exact problem on my own printer.
You have a perpendicular, or parallel, axle (x,y,z) problem, they are not at exact 90 degree, or parallel, from each other.
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