how to fix the white page in system configuration - admin panel of Magento

 Problem: in the admin panel, If you go in system->configuration, you find the (content part) page empty (the header, menu and footer are visible).

Before anything check your logs in /var/log/.

Try to refresh all the magento caches (system->manage caches)
If it doesn't work edit /app/etc/local.xml and disable the local modules setting th "disable_local_modules" to true - <disable_local_modules>true</disable_local_modules>.
REMEMBER to "flush magento cache" (system->manage caches) or disable the caching before reloading the system->configuration page or doing any other operation.
If the message "access denied" appears you don't have enough authorizations for a specific module.
To solve  go to system->permissions.>roles (choose administrators)  and click on "save role" with permission to "all" selected.
In older versions of magento or if you haven't (or can't) rebuild you need to set the system->permissions->roles  (choose administrators) to "custom" and, after selecting everything, save the role.

For a permanent fix (to choose "all/everything") you need to modify the /app/etc/config.xml file and add the users group that is missing.