Luca,
SDK does support IE9, it looks like you don't have the same version in between SDK and Plugin. I strongly suggest you to use latest version of SDK and plugin which are in version 2.1.0.60772. You can find those under
http://developer.cisco.com/web/jabber-developer/get-started1Please make sure to uninstall the old plugin and the restart might be required.
Hope this help
Howard