I know this is an old thread but we had a similar problem. It turned out that when we ftp'd the site from one server to another, the ftp client had a limit on directory depth. This in turn missed some of the php libs and the require_onece(); statements failed leaving us with a blank page.
Namely: lib/xmldb/classes/generators/XMLDBGenerator.class.php
Installing and upgrading help
ADMIN => Blank Page
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