I ran into this issue in my vSphere 5.5 lab. After starting up my vCenter Server Appliance, and attempting to log in using the web client, I received this error:
This error prevented the login to vCenter. After a quick search, I found this knowledgebase article, which describes toggling the option to regenerate certificates:
After rebooting the vCenter appliance, however, I still received the same error. After digging around a little bit, it seems that the certificates are regenerated if the IP address or Hostname of the appliance is changed. With this in mind, I temporarily changed my IP address, rebooted the appliance, and the certificates were regenerated, which then allowed me to log in.