403 Forbidden error on some admin settings

I apologise first for creating another thread on this issue, I've searched and tried the solutions available in different discussions but to no avail :( Basically I am using Drupal version 6.13 and am getting 403 Forbidden errors when applying some administration settings but not on everything.

The best example is when I apply a setting to the admin menu, I get the following:

You don't have permission to access /drupal/admin/build/menu-customize/admin on this server.

Drupal is installed on wwww.domainname.org/drupal
There are no folders called admin in the root of the drupal folder

I've made sure that .htaccess in the drupal folder has:

Options +Indexes
Options -MultiViews
Options +FollowSymLinks

And now am totally stuck, any suggestions at all would be much appreciated. I've included a copy of .htaccess file

My .htaccess file is:

#
# Apache/PHP/Drupal settings:
#

# Protect files and directories from prying eyes.
<FilesMatch "\.(engine|inc|info|install|module|profile|test|po|sh|.*sql|theme|tpl(\.php)?|xtmpl|svn-base)$|^(code-style\.pl|Entries.*|Repository|Root|Tag|Template|all-wcprops|entries|format)$">
  Order allow,deny
</FilesMatch>

# Don't show directory listings for URLs which map to a directory.
Options +Indexes
Options -MultiViews

# Follow symbolic links in this directory.
# Options +FollowSymLinks

# Make Drupal handle any 404 errors.
ErrorDocument 404 /index.php

# Force simple error message for requests for non-existent favicon.ico.
<Files favicon.ico>
  # There is no end quote below, for compatibility with Apache 1.3.
  ErrorDocument 404 "The requested file favicon.ico was not found.
</Files>

# Set the default handler.
DirectoryIndex index.php

# Override PHP settings. More in sites/default/settings.php
# but the following cannot be changed at runtime.

# PHP 4, Apache 1.
<IfModule mod_php4.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   0
</IfModule>

# PHP 4, Apache 2.
<IfModule sapi_apache2.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   0
</IfModule>

# PHP 5, Apache 1 and 2.
<IfModule mod_php5.c>
  php_value magic_quotes_gpc                0
  php_value register_globals                0
  php_value session.auto_start              0
  php_value mbstring.http_input             pass
  php_value mbstring.http_output            pass
  php_value mbstring.encoding_translation   0
</IfModule>

# Requires mod_expires to be enabled.
<IfModule mod_expires.c>
  # Enable expirations.
  ExpiresActive On

  # Cache all files for 2 weeks after access (A).
  ExpiresDefault A1209600

  # Do not cache dynamically generated pages.
  ExpiresByType text/html A1
</IfModule>

# Various rewrite rules.
<IfModule mod_rewrite.c>
  RewriteEngine on

  # If your site can be accessed both with and without the 'www.' prefix, you
  # can use one of the following settings to redirect users to your preferred
  # URL, either WITH or WITHOUT the 'www.' prefix. Choose ONLY one option:
  #
  # To redirect all users to access the site WITH the 'www.' prefix,
  # (http://example.com/... will be redirected to http://www.example.com/...)
  # adapt and uncomment the following:
  # RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
  # RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]
  #
  # To redirect all users to access the site WITHOUT the 'www.' prefix,
  # (http://www.example.com/... will be redirected to http://example.com/...)
  # uncomment and adapt the following:
  # RewriteCond %{HTTP_HOST} ^www\.example\.com$ [NC]
  # RewriteRule ^(.*)$ http://example.com/$1 [L,R=301]

  # Modify the RewriteBase if you are using Drupal in a subdirectory or in a
  # VirtualDocumentRoot and the rewrite rules are not working properly.
  # For example if your site is at http://example.com/drupal uncomment and
  # modify the following line:
  # RewriteBase /drupal
  #
  # If your site is running in a VirtualDocumentRoot at http://example.com/,
  # uncomment the following line:
  # RewriteBase /

  # Rewrite URLs of the form 'x' to the form 'index.php?q=x'.
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteCond %{REQUEST_URI} !=/favicon.ico
  RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
</IfModule>

# $Id: .htaccess,v 1.90.2.3 2008/12/10 20:04:08 goba Exp $