This is widespread error; the image might have been uploaded to the server by FTP in a mode different from binary. In some FTP client programs, the default transfer mode is text-based (ascii), where any sequences of “end of line” characters are replaced by the ones used in the client’s system. In this case, the boot disk image is bound to be damaged. Check the SHA-256 checksum of the received file: if it differs from the one available on the FTP server, the error must have occurred during the transfer process.