Vcenter Operations Manager error – This product is unlicensed or cannot connect to vSphere Server

This one is easy but I just want to post this because sometimes it’s just there in the KnowledgeBase…
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2001466

Why ? This happens when changing Serial numbers or changing from a trial to .. official license or… another trial 🙂
Solution summary : unregister the vcenter and register it back again