File Uploading (Http Error #403)

http://www.kirupa.com/developer/flash8/uploading_fileReference_pg1.htm

I was using the script on the above page to upload files and I get 403 on my mediatemple grid server.
(It works fine on localhost)

I tried adding the following to .htaccess:

SecFilterEngine Off
SecFilterScanPOST Off

…but it gives me Internal Server Error - 500

Any help would be appreciated.
Thanks
-Miro