Usually you get this error when the script you're trying to run isn't owned by yourself or it's permission is set to be writeable by users other than just yourself and you're running under suPHP restrictions. Try changing permissions so that the directory is set as '755' instead of '777' often suggested by some 3rd party software.
See also the following knowledgebase article on suPHP and use of .htaccess.
http://www.aegishosting.co.uk/support/knowledgebase.php?action=displayarticle&catid=6&id=8