After setting up a new WordPress site (no plugins/themes installed or any other modifications; just WordPress “out-of-the-box”), I recently ran into an issue where I could not update/save any changes made in the General tab (under Settings). No matter what I tried, I would receive a “Forbidden You don’t have permission to access” message; however, I did not encounter issues updating/saving any of the other areas (e.g., Reading, Writing, Discussions, etc.). In looking at a few forum posts, several people suggested that you should check to ensure folder permissions (specifically wp-admin) are set to 755 and file permissions (specifically options.php and options-general.php in my case) to 644 – check and check. NOTE: If you have a similar issue with the permalink tab, for example, you might check options-permalink.php instead.
The next troubleshooting tip actually worked. A member suggested an issue with mod_security and to update the .htaccess file. Sure enough, I added the following (below mod_rewrite in my .htaccess file) and was immediately able to save changes to Settings -> General:
<IfModule mod_security.c> SecFilterEngine Off SecFilterScanPOST Off </IfModule>