"Edge not recovered, polygon fill suspect" with "shortest distance on surface"

I am getting the following error when marking up on a surface/model:

[VTK] vtkDelaunay2D (0000024030965F60): Edge not recovered, polygon fill suspect

[VTK] - Warning: Warning: In D:\D\P\S-0-build\VTK\Filters\Core\vtkDelaunay2D.cxx, line 1409

It is very reproducible with my own hi-res CT data, but I would not reproduce it with the sample CT data (head and heart). No problem with spline measurements.

Any idea what the error message mean or how to troubleshoot?