Ultimaker 3 users sometimes encounter an error that says “Difference between detected height of both PrintCores exceeds realistic values”. This is especially common during initial setup, or when the active leveling function is used.
This error can be caused by several things, but the most common cause is that the bed is positioned too high, so when the bed is raised, the nozzles are hitting the bed sooner than expected. The fix is a simple matter of lowering the bed position using the three adjustment screws on the underside of the build plate.
Here’s a short video to show how to deal with this error: