look for a missing texture.
that happens to me every once in awhile, like when loading a group i saved long ago. or sometimes i accidentally overwrite/replace/rename a texture, and dont switch it over. go into texture mode, and select missing from the texture categories list. itll show up there if thats the problem.
if that doesnt work, copy and paste your build report, and packfile creation report here, and we\'ll(or i'll)have a look see.