That is a pretty small amount of layer shift so I'm skeptical. Anyway layer shift can be caused by either excessive friction or loose pulley screw. Is this X axis slipping? Obviously only one axis has the problem. Usually the slip is at the stepper motor or the pulley above (the 2 pulleys on the short belt). Most often the motor pulley as this has the most force on it.
Tighten the hell out of those 2 set screws. The shaft of the tool should twist. If you got your UM from ultimaker then it should also have come with a spare set of silver colored screws (the set inside the pulleys are black). The silver ones are pointier and better. But not mandatory.
Check that the short X belt is not rubbing the wood. Watch the short X belt while printing. Does it twist one way when moving right and the other way when moving left? If so it is rubbing the wood slightly. Fix this by moving it away from the wood. Either add one washer under each of the 4 plastic spacers or move the pulley closer to the motor (as close as possible without touching - about .5mm).
Finally, like Tommy says above, check resistance. You should be able to easily move the head with your smallest finger of each hand on opposite blocks.
You may want to check that your x/y axis are properly aligned perpendicular to eachother (forming a perfect cross), allowing the printhead to move freely on both axis... With the printer turned off you should be able to push the head on both axis with one or two fingers, without too much force.
Also check your electronics cooling and make sure your stepper drivers are not getting too hot (they will miss steps) how hot does your stepper motors get when printing?
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