The version 5.1 vSphere Web Client fails to connect to a version 5.0.x vCenter Server.
This problem is accompanied by the error message Failed to verify the SSL certificate for one or more vCenter Server Systems: vCenter_5.0_IP
This problem occurs if the version 5.0.x vCenter Server is registered with a version 5.0 vSphere Web Client without accepting the SSL thumbprint, and then the vSphere Web Client is upgraded to version 5.1.