It's not working in the dashboard only, the front-end works like a charm.
I understand that it could depend on a conflict with another plugin. Is there any way to quickly understand which one without disabling them one by one (it's a production website with some dozens plugins...)-
http://wordpress.org/extend/plugins/wpsc-support-tickets/
without disabling them, you could use Firebug to see what javascript issues are popping up, and that should trace back to a specific line of code within a javascript file, which should also reveal the plugin causing the issue.
