If that doesn't work, then I have no ideas.... 1.7.8 fixed one known bug that triggered that particular error.
Besides the fact that the active sheet index should be an integer (though using a string containing a 0 shouldn't cause this error)
Can you try dumping the results of an array_keys($objPHPExcel->getAllSheets()); and $objPHPExcel->getActiveSheet(); immediately after the load.