Please go to the Client Area and download a fresh set of files then extract these to your local computer. Once done, upload these to your server, overwriting what is there.
Unfortunately, that file is either modified, out of date, or inaccessible by PHP. If the permissions are good on the file per your server's configuration, you will need to ensure it is not modified or out of date. I'm afraid, there is no other way to get around this safely.