Enable error logging by placing this in your settings.php :
error_reporting(E_ALL & ~E_NOTICE & ~E_DEPRECATED); ini_set('display_errors', TRUE); ini_set('display_startup_errors', TRUE);
This may give you the best error information you can work with.
EDIT: As Clive noted in the comments, you should add this to the beginning of index.php so that these error report options are used for any errors that occur even before settings.php .
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