I know this is an old issue, but I'm having the same issue in Moodle 1.7+. I've added the code by Ari Tahvanainen (found in MDL-6494) into the backup.php file. It now somewhat gives me the following message "backup functions is/are missing", but doesn't tell me which module might be the culprit.
I'm assuming this is no longer an issue in newer versions of Moodle, but I won't be able to upgrade until the summer.
Any help would be greatly appreciated.