This post should be called instead «Making Opera work around a Cisco Secure ACS 4.1 interoperability bug».
Continuing with the stream of web browser interoperability issues I recently found how to make Opera 10 work with a Cisco Secure Access Control Server 4.1.
The problem I experienced is briefly described as, clicking any option in the menu (in the left frame) will open the menu again in the right frame, instead of actually showing the selected option.