this the most frequently encountered error with iso2mesh (or tetgen). It is very likely that your original mesh contains self-intersecting elements, and mesh simplification (decimation) usually won't fix it (If you are using FreeSurfer, the pial surface it produced usually have this type of issue).
There is a work-around, but you might have to play with the options to get it to work. Please look into the sample script sample/demo_remesh_surface.m. It calls a function named remeshsurf in the toolbox. What this function does is to first convert a closed surface into a binary image, and then re-extract a surface from the volume, and the extraction script guarantees that the produced mesh is self-intersection free.
If your mesh is complex, you may have to use a large volume to re-voxelize your surface. Give it a try and let me know if it works.
Error: Invalid PLC.
Two subfaces (17561, 18027, 83603) and (83474, 83572, 18027) are found intersecting each other. Hint: Use -d switch to find all intersecting facets.volume mesh generation is complete
I am not sure the reason for this message and also if there is any potential work around it.