First obvious thing to rule out is that your definitely uploading the form.tar file and not the zip you downloaded? Secondly the error code seems to suggest an issue with the PHP Phar extension. Can you check with your host that it's enabled and working? Worse case if your host doesn't support Phar, I can always give you instructions to manually upgrade an app via ftp.
I assume by the above that your not hosted by IPS and have ftp access to your files? If so, I can give you an error to patch the above bug until the next version is out.