What about generated error log files in your logs folder? Don't you possibly have any myDEbug-adm logs? Not guaranteed for a 403, just asking...

Nothing in the above posted copy specifically stops the configuration.php file especially if able to access other menu options. Now there is I think it was a possibility that the database server has been upgraded and now items in the configuration table are no longer as they should be to support the expected operation and so therefore coming across this issue... just guessing though in large part.