First of all, add the following line to your local configuration file (system/config/localconfig.php):

$GLOBALS['TL_CONFIG']['displayErrors'] = true;

If there still is no error message, you are probably running a PHP version prior to 5.1 or you are missing the PHP mbstring extension. If you are using the Safe Mode Hack, make sure that your server supports FTP (compile PHP with --enable-ftp).