I got this email today: I figured it out! I DLed some Bryce mats tonight, and I got the same message when trying to unzip them. They were .sit files, which had never given it problems before. I noticed, however, that they were named like your files, i.e., whatever.mat.sit. When I renamed the files, by removing the .mat part, they worked fine. I looked in my recycle bin, and found hoodoos.obp.zip. I restored it, renamed it, and it unzipped just fine. I don't know why my PC is so picky, but that was the cure.