I had an odd white screen on one of my websites. No exceptions, no messages, it was time to check the error log.

There I found the following error:

PHP Fatal error:  Call to a member function extend() on a non-object in httpdocs/lib/Varien/Simplexml/Config.php on line 600

That looked very scary like there was some mess with the core library of Magento.

Actually it turned out it’s not that bad just I had a malformed XML file under my “app/etc/modules/” directory.

Removed the file, you might need to use a bit of a trail and error here, and all worked as a charm.

Also make sure that you have there only XML files with “.xml” extension.