No permission to upload downloadable products on marketpress

My downloadable products had been working just fine a few months ago. I recently added WPMUDEV's Appointments + and the slide in plug in, and have noticed that I cannot now upload files to any new products. I get this error: Forbidden

You don't have permission to access / on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Apache Server at Port 80

We check permissions on the file folders on the file manager and made the upload folders 755, and we still get the same error when trying to upload a file to the product. I need downloable products to work again!