Marketpress 500 error trying to upload media product

When I click "upload file" when editing a post to add a product download, I get a server 500 error:

"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."

This is a new install, just configured. The permissions on media-upload.php are 644. Trying other options, up to 755, did not change anything.

Help!