The error may be caused by the following issue on your server,
1. Invalid .htaccess file contents. The script does not come with this file in the admin folder, please check it on your server.
2. Incorrect folders or files permission. If you are using regular CPanel hosting, you can ignore it since mostly no problem with it.
3. Your server may be using mod_security. Generally, it causes an issue when url or html tags are submitted. If so, try to disable or adjust the settings to allow form submission on your site.