I did use admin/admin originally. However, I did get it to work. It appears the admin account was locked out?????  I attempted to login to the admin CLI (ssh cluster IP address) and login with the admin/admin account failed. I then ssh to the Cluster IP again using the security-admin/security-admin account and ran the command unlock_user.  This then allowed me to attempt to run your command you posted... update_vcenter_server_registration . However it was requesting a VCenter Server name and said it did not exist. Running the command get_vcenter_server_view came back with nothing confirming there were no VCenter Servers attached.  So I decided to manually add the VCenter Server in the Admin CLI using the command register_vcenter_server and supplying the requested information at the prompt (Server Name, vcenter IP, vcenter port, username, password, and certificate). I got the response that the "vCenter Server registered at RPA Cluster successfully."  In previous troubleshooting, I had manually added the RecoverPoint Web Client plugin into the directory C:\Program Files\VMware\vCenter Server\WebClient\plugin-packages and restarted the WebClient Service and this allowed the RecoverPoint plugin to show up in the WebClient.  After manually adding/registering vCenter Server in the admin CLI above and restarting the Web Client service, the Plugin is now showing correctly in the WebClient and I can manage and ProtectVMs.
Trying to setup RP4VMs for testing and getting an error on the final step within the Deployment Manager.  Failed to set the given vCenter Server Credentials  During this stage the cluster IP becomes active but it fails with the above message.  I am using the administrator[NAME].local account and able to successfully select which datastore to place the RP Repository and I have confirmed the repository is created on that datastore. There is no question about the credentials.   I am running the deployment manager on the vCenter Server itself and no windows firewall on Server 2012 R2   RPA cluster and vCenter Server at Secondary site is working fine with no issues... same setup other than Windows OS (Server 2012).  Attached dm.log file
RP4VM Failed to set the given vCenter Server credentials