I found this string in the forums:
and I'm experiencing the exact same symptoms, but the site is not running Bulletproof Security so they suggested I start a new string. This quote from the other string describes the issue perfectly:
"When I try to upload a digital product, I receive the following error:
You don't have permission to access /wp-admin/media-upload.php on this server.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
I followed all of the troubleshooting discussed in that string, to no avail. HALP!!!