random gaps on top layers / Cura 2.6, Ultimaker 3

[hlinden 0]

I've got some trouble with our Ultimaker 3 - in the print below, I had three kinds of cylinders and I had cloned each one three times, leaving the arrangement of all twelve pieces on the print plate to Cura. The result is, that randomly two of the cylinders are developing weird patterns while the rest is fine:

Is there any explanation for this behaviour? Note that this print is not finished yet, I'll post a picture of the final result tomorrow.

[hlinden 0]

The solution for this one as well was, that the meshes we got for printing had inner faces and vertices. This apparently leads to very random behaviour by Cura, where the layer preview looks neat and regular, but the print isn't.