....what puzzles me is if the belts etc are slipping then why not show that fact at the beginning of the print and not wait until it reaches a height of around 20mm.
Perhaps the conditions that cause the actual slipping to take place don't happen until the print reaches the 20mm height! And it may not be the belt slipping....it could be the stepper motor "slipping" electrically.
What speeds do you have in your settings? Both printing and movement!
Have you tried slowing down the printing and/or movement speeds to see if the slipping stops?
Is there anything at or near the 20mm height in the model you are printing that would change the printing or movement speed?
Is there anything significant about the part at or around the 20mm height? A change in shape, etc
Is there any noise of any type from the printer at the point the layer shift takes place?