Don't really know why it would do this only with one file, but have you tried canceling the download and deleting the incomplete file. Then try searching for it again and redownloading it. Use Bitzi to check the file
Right-click on searchresult and choose Advanced>Lookup File with Bitzi

It could be a bad file or maybe the downloads.dat file in the incompletefolder got currupted in some way (very strange that it would just be the one download if that was the case)