Layer skipped on z axis
@gr5's advice is good, but whatever you end up doing to fix it -- my guess is it's the Z-linear bearings, because the same thing happened to me -- make sure to contact your Ultimaker reseller if your UMO+ is still under its 1-year warranty. I replaced my Z-bearings at my own expense and later found out they would have been covered under warranty.
As you may have guessed the Z axis is probably slipping (moving too far) such that the next layer is severely underextruded (not enough inill).
Does it always happen at the same heights with the same patterns? If so it's probably Z rods, Z bearings, Z screw, or Z nut. But I would also suspect the Z driver.
Put a heavy brick or lead weight or something on the bed towards the back. Reprint something. If the gaps get much larger then your z driver is shutting off due to overheating. The problem gets better then it's the mechanics I mentioned.
Start with the easiest - clean the z screw completely in the area where it fails (also try to feel for disontinuities). If that doesn't fix it and the break was helping then you need to take it all apart. I recommend replacing the Z nut first. That's like $1 for a much better nut than the default nut. While that's apart you can feel if there is anything wrong with movement when the z screw is removed. If so then replace the rods and z bearings.
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