There's a new bug on SourceForge that affects mirror selection. If you've manually selected a mirror that no longer exists, it will fail to download. If you don't have a SourceForge account, you can clear the selection by clearing your browser's cookies for sourceforge.net.
If you do have a SourceForge account, you will have to log out to be able to download. There is no way to select auto or a different mirror.
Details here: http://sourceforge.net/p/forge/site-support/8935/