What does error "Can't read LFN File - Error - Already on File" mean?

LFN stands for Logical File Name. If you use symbolic links or the "Use DB" feature, many Logical file names can map to the same physical file. A logical file name is (roughly) an application ID + database ID + filename. You can usually get rid of the error by exiting Appx and going back in. It wouldn't hurt to verify files as well.