After upgrading to VMware vSphere 5.1 All images backups returned with a No Data error. So there is/are no backup of the client(s). So what are we dealing with here? Simple. vCenter Single Sign-On is a new feature of vSphere 5.1 which is causing this problem. EMC Avamar is not authorized to make connection to the vSphere server. SSO is not just an authentication broker but also a security token exchange providing a more secure way of accessing your vSphere solutions. What that means is that when you previously logged into vCenter Server, you were authenticated with the provided username and password against the Active Directory configured for vCenter Server. With vSphere 5.1 and vCenter Single SIgn-On you no longer log directly into vCenter Server but with a security domain defined by your vSphere environment. When logging in to vSphere 5.1 you actually pass authentication to the vCenter Single Sign-On server which can be configured with multiple identity sources like Active Directory and OpenLDAP and on successful authentication, your username and password is exchanged for a security token which is then used to access the vSphere components like vCenter Server and vCenter Orchestrator etc.
Start your EMC Avamar Administrator and Click Administration
Right Click on your vCenter server and Click Edit Client
Add you domain before the vCenter account. After Ok it should be able to connect again.
Restart the VMware vCenter Connection Monitor from the Avamar Administrator GUI (Administration / Services Administration). Run a manual backup to see if it work