When I had the same error something like this: before error after error I was able to fix it by changing the material from Matcap Red Clay to Matcap Red Wax.
I had a similar issue where no matter what texture I turned off, mask cleared, or material I chose, the whole model appeared very dark. For me it wasn't a transparency issue either. I solved it by opening zbrush and creating a primitive. The default primitive loaded the materials correctly, and then from there I reloaded…