If you are experiencing issues during uploads with the Batch Uploader (aka. Drag and Drop uploader, Flash Uploader), take a look into your browsers error console.
When you are accessing the Dashboard with HTTPS, its most likely a mixed content error, i.e. the uploaders tries to sent the file using an unsecured connection from a secured site. This is often caused by the WordPress HTTPS plugin ().
Here’s the easy fix: add the following to the “Secure Filters” in the HTTPS plugin settings: