again the same problem, circles are not round

hi there, after having a lot fun with my sd2 i printed some gearwheels and i found out that they are not perfectly round. i excavate some of my first prints and i saw that all the holes that should be round were a bit oval, with a flat on the upper right and lower left side.
i scearched through this Forum and found out that tightening the belts would fix this Problem but i can tighten or loosen all the belts, there seems no difference. i also realigned the pulleys on the y rod at the back of the sd2 but no difference. could it be possible that the y stepper Motor is damaged or not perfectly configured (current, steps per mm)? or did i just not realigned proberly? when moving the carriage Forwards and backwards, the rod wont move to the left or right, so i think this should be perfect(?)

i attached some pics where you can see the Problem.
in the second pic you can see one outcome of this Problem, the infill does not reach the Perimeters and therefore the plastic wont stick together in this area.

would be great if anybody have an idea