1. The most common cause of this error is you delete the public_html folder which is your web site root folder that contain your web site files and folders.
2. You might accidentally change the permission on public_html to not allowed everyone or world read access. The actually permission setting will depend on your host provider web hosting setup and requirement.
3. You might have a bad .htaccess that have incorrect rewrite rule that cause it to redirect to none existing page, etc
4. You might use the file with the wrong name or wrong case or point to a missing file/folder, etc.
Type of error:
. If the error is immediate upon access your site it is very likely cause by item 1,2 and might be 3
. If the error only occurs when a link is click then it is very likely cause by item 4 or might be 3
If you fix the error then the redirect to error page will not happen.