I've gotten this a few times on a couple of projects. Sometimes the materials are not properly linked to the name in the file, you may have moved the image to a different location or the image itself is corrupted, try assigning a new image to the materials or renaming the materials and applying the images again. If you ever get a message like this when you open a rvt file, save it under a new name and it should be alright. Hope this helps.
