You are unable to download a quarantined file while accessing the administration console over https. This occurs when you use Internet Explorer as your browser.
Sophos product and version
Sophos for Microsoft SharePoint, v 2
All supported operating systems
What to do
In Internet Explorer, go to Tools | Internet Options | Advanced, and deselect "Do not save encrypted pages to disk".
According to Microsoft documentation this option is enabled by default in IE8. When this option is enabled, IE cannot store the item in the Temporary Internet Files cache.