You need to provide more information really. As @Imaginaerum mentioned, there is no import/export feature for attributes in standard Magento CE 18.104.22.168 so you must be using an extension. Without telling us which one, there's no way we can give you specific help.
That said, one golden rule that will help you with this sort of error: If you're getting a non-Magento-branded error page such as "Internal Server Error" then it's an error Magento has failed to catch and the error message will be in your web server log file (e.g. /var/log/nginx/error.log or /var/log/apache2/error_log), not in Magento's log files.