Had the exact same issue. I had two images with a colormap internally stored. Tried "Delete Colormap"; didnt delete it... So I thought hey maybe I should just delete and rebuild everything. So I tried "delete attribute table". Got a red x and an error 000114: Failed to delete attribute table. Read the error message details and it suggested deleting the xml.dbf file associated with it. Once, I delete the two .dbf files for my two images, the mosaic tool worked fine for me.