Recently, I tried to do an XML Export of this blog to backup my posts, pages, categories, tags, authors,  and more for safe keeping in the event I lose all my posts because of a web hosting, plug-in problem, or I accidentally messed up my database and all my posts.
When I tried to export my XML file with WordPress Export Wizard. I downloaded a zero byte XML file which was empty when I opened it with WordPress.
Another good reason to have an XML export of your blog is if you plan on using free hosting to host your blog like wordpress.com free plan since wordpress.com does not let you access the database for wordpress.com for its free members, so you have to use the XML backup file.
Plus, if you are using an old database with lots of old tables and orphaned data from plug-ins which were uninstaller, your database can be too big to upload. Using the XML import wizard is the easiest and most safe way to transfer your blog to a new web host after you installed WordPress on a new web host.
1. Deactivate all your plug-ins for WordPress Plugin section in WordPress Dashboard, and switch to the default WordPress theme which is TwentyTen for WordPress 3.0.
2. Try XML Export again to see if it works.
XML Blog Export work for me after I deactivated all my Plug-ins for WordPress.
3. If XML export still does not work, you can try reinstalling WordPress by hitting the Updates link in the Dashboard settings or reinstalling WordPress Manually.
4. You can also try optimizing and repairing your WordPress Database in PhpMyAdmin. Backup your database before you optimize and repair your database file.
Now XML Export should work for WordPress!
More information is available at http://codex.wordpress.org/Updating_WordPress for updating and reinstalling WordPress.
If you like, you can test out the XML file on a newly installed WordPress blog on your server or free web hosting like WordPress.com.