You have a MiTM SSL proxy installed on your machine. That means some software on your machine is intercepting all secure traffic un-encrypting it and then re-encrypting it with its own certificate. This is extremely dangerous, it means some software on your machine can read all your secured communications. Commonly these are used to inject ads into HTTPS traffic, but some idiotic antivirus programs also install them.
If I were you, I'd wipe the machine and install a new copy of windows straight from Microsoft.
All calibre communication with code.calibre-ebook-com is using a private certificate, so it will fail when a MitM proxy is installed. You can always download the plugins manually using a browser from
http://plugins.calibre-ebook.com and then install them in calibre by using the Install from zip file button.