This warning appears due to the track navigation in the geometry since track is formed as of small steps and if in the geometry the size of voxel is much smaller than track experience zero steps. This is only warning messages, and it does not affect results.
BTW, i once fixed this problem by making changes in the main G4 code, i.e. look for the G4Navigator.CC and make changes in
Step += 100kCarTolerance;
by
Step += kCarTolerancestd::pow(10.,fNumberZeroSteps);
The original author of this sugestion is Pedro Acre
Ref.: Geant4.10: G4Navigator.cc Source File
[Look line 954.]
Ref: DICOM example Stuck Track error for small voxels - #5 by tgilat
VRS