I seem to mainly get that error message when I've ticked the "other materials" box and there is a material in a prop or figure (e.g. hair or jewellery) parented to the figure maybe that isn't recognised... or perhaps the texture set I've applied has extra material zones in it and those have been added to the figure.

Also had it when I've accidentally applied an inappropriate material set to a figure, adding the material zones from an unrelated prop, and have then gone to run EZSkin.

In all these cases it should be okay to simply tick the ignore errors box... and thereby avoid okay'ing lots of erroneous messages.

Cheers