The file may really be corrupted, or there may be an old Loader installed that does not recognise the type of encoded file that you have.
You can check whether the Loader is already installed by using a phpinfo script, and if a version 4 Loader is installed, e.g. 4.X, or lower then that should be updated to the latest Loader. The latest loaders are available to download here, you can also sign up for Loader release updates on the downloads page.
Corrupted files are rare, but can occur to some types of encoded files if they are uploaded using an FTP program in ASCII mode instead of binary, or if the TAR smart cr/lf conversion feature is enabled in WinZIP.
If updating Loaders does not resolve the problem, try reinstalling the PHP script making sure to transfer in binary.
