Marketpress won't allow me to upload a product file

I found this string in the forums:

https://premium.wpmudev.org/forums/topic/marketpress-doesnt-allow-me-to-upload-a-product-file?replies=20#post-491836

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:

[code]
Forbidden

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.
[/code]"

I followed all of the troubleshooting discussed in that string, to no avail. HALP!!!

Thanks guys.