Hello,

In this case maybe you have some special rules in your .htaccess file or your Apache virtual host, I would suggest trying with the default Wordpress .htaccess rules and see how it goes:

# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress

Another thing that might be causing the problem is an Apache module like mod-evasive for example. Could you run the following and check for any special modules like mod-security and mod-evasive:

apachectl -M

Also make sure to check your Apache error log for more information:

tail -100 /var/log/apache2/error.log

Regards,
Bobby