Seems that you have not registered a domain user or a group during the upgrade of the vCenter. The screen where you need to remove administrators and add domain/group or domain/user. Refer to the following article which I wrote
http://vxpresss.blogspot.in/2013/03/vsphere-51-single-sign-on-sso.html
Point number 14 - Register a vCenter Server administrator user or group with vCenter Single Sign On. This is the group which will automatically get added to SSO.
This will allow you to login to the Webclient using an account which has permissions on SSO and also has access rights to the vCenter Server.
Cheers
Sunny