When attempting to protect or re-protect an agent, authentication fails. The message may resemble:
DEBUG 2016-04-28T15:03:14  - Replay.Core.Web.Helpers.LogErrorAttribute ()
Exception encountered at routine: ProtectionWizard/Connect
WCFClientBase.ClientServerErrorException: Call to service method managementservice:Replay.Core.Contracts.Agents.IAgentsManagement ValidateAgentProtectionAbility failed: The connection with '192.168.155.202' cannot be established. Please, ensure that the specified user credentials are valid and have administrative privileges ---> Replay.Core.Contracts.Agents.AgentConnectionException: The connection with 'Agent name or IP address' cannot be established. Please, ensure that the specified user credentials are valid and have administrative privileges.
You are able to log in with administrative rights via RDP onto the agent machine, so you know that username and password are correct.
Verify that the time are synchronized between the core and agent. If time are synchronized and authentication still fails, make sure that both machines are configured with the correct time zone. Too large a time difference will cause non-domain authentication to fail as well as domain authentication.
If one or both of the machines is not on a domain, consider syncing to an external time source, such as an NTP time server. Please see https://technet.microsoft.com/en-us/library/cc773263.aspx for help in configuring Windows to sync to an Internet time server.