Magento: Page won't stop loading in admin/system_config/edit/section/design-Collection of common programming errors

I was using Magento 1.4.1 and upgrade gracefully to 1.4.2. After testing if the upgrade was Ok, I made some modification in order to have a new home page layout for a store using these instruction. The modifications have been tested on a local version (Ubuntu 10.04, php 5.3.2), and worked great.

When I upload the files to the pre-prod server (Centos 5.5, php 5.2.14), and access the System->Configuration->design tab in the admin backend, my browser seems to keep loading indefinitly.

What I have done:

• I copied the app/code/core/Mage/Page/* directory to the app/code/local/Mage/Page/;
• I created the app/etc/modules/Mage_Page.xml, and defined the codePool to local;

What I have already checked:

• I got no errors in /var/log/httpd/*.log;
• I got no errors in magento’s var/log/system.log;
• The frontend works fine;
• The backend can be accessed, except the System->Configuration->design tab;
• I tried to revert my modifications to a revision before the modification were made (using svn);
• I cleaned up the cache using rm -fr var/cache/mage* var/session/* directly on the server;
• I restarted the server multiple time;
• I even tried to get a dump of the db in production on the preprod server, with no effect. I still got the same issue.

If someone could point out anything I could have forgot. I am ready to try anything to make it work, since it would not affect the production server for now.