Were the tiny screws really tiny? Like grub screws for the pulleys? It could well be that some of the pulleys are still loose, so that the head doesn't move accurately.
There are 11 pulleys - 9 easily visible, and two on the motor shafts, that you can only see by removing the motor covers (each of which is held in place by one screw about halfway up the left panel (towards the back) and the back panel (towards the right). Check that all of them are tight... in particular, try grabbing the four thick axis rods that are just under the edge of the cutout in the top panel, and try to shake the rods back and forth along their length. They shouldn't move. If they do move, it's a sign that the pulleys aren't tightly attached.
Also, listen for any odd screeching noises as the head moves - the motors or belts might be catching on the frame, or the thinner x-axis cross rod (that goes through the print head from left to right) can also catch on the side panels.
Yes! This is very common. Not as common on the UM2 though. There is a whole thread about this. If it is a gentle/gradual lean then it is always a "rubbing" issue. If it is sudden jumps on random layers then it is a loose screw issue. I can't find the thread but I know it's one in the last few weeks and it has photos of the problem and how it was fixed.
Anyway - if it is leaning left or right then it's the X axis - the left/right axis that has the problem. The short belt and/or the motor is rubbing the wall. Or the friction is excessive.
Turn off power to the UM2 and push the head around by hand. Make sure the friction is similar. The X axis stepper motor should be in the rear right corner.
Remove the white metal cover over the stepper. To remove the cover look at the edge where the cover touches the back and side - somewhere towards the middle is a single welded threaded cylinder. Look at the same spot on the outside of the UM2 and remove the screw - now the cover comes off.
Either the pulley on the stepper is too close to the wall, or the pulley that this "short belt" goes to at the top is too close. Decide which is more likely - I'm guessing the motor. If the motor you have to remove the 4 screws holding the motor in place, loosen the pulley on the motor and move it a tiny bit closer to the motor. As close as possible without touching the motor - typically 1/2 mm is good.
That's probably it.
Alternatively the pulley up at the top of the short belt might need sliding slightly away from the outer wall - not sure if that is adjustable actually. So it's probably the motor.
Look for black powder in that corner also - if the belt is rubbing against the back panel it slowly turns to dust.
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