This error can appear after active leveling. It indicates that the measured Z offset of the two print cores is larger than expected.
There can be several causes for this error, but it is most likely that there was a problem during the active leveling procedure. To resolve this issue, follow the active leveling procedure again. Pay attention to the following:
Make sure that your nozzles are clean
Material that is stuck to one or both of the nozzles can influence the readings. Although the nozzles are heated up before the procedure begins, if a blob of material is too big, it will not melt away completely and will add to the height of the print core.
Before restarting active leveling, clean the outside of the nozzles as described on this page.
Make sure that your glass plate is clean
When a nozzle is pressed against the glass during active leveling, this can leave behind some filament residue on the plate. This residue can cause inaccuracies in the readings if you go through the active leveling procedure again. Make sure there are no impurities on the surface of the glass plate.
Note: a thin layer of glue correctly applied to the glass plate will not affect the readings during active leveling.
Make sure that the bed is not set too high
For active leveling to work correctly, there should be at least 0.5 mm between the tip of the nozzle and the glass plate at the start of the procedure. If the nozzle is already touching the glass, the Ultimaker 3 cannot perform the measurements correctly.
After having followed the manual leveling procedure multiple times, the build plate springs can become too loose and the bed too high. Turn the three knurled nuts counter-clockwise to tighten the screws. There should be a distance of approximately 14 mm between the bottom of the heated bed and the top of the aluminium base plate. Now manually level the build plate once more to ensure that it is leveled correctly before trying active leveling again.
If you are still experiencing problems after following all these instructions, please raise a support request above.