Symptoms

• Cannot connect to the ESXi 5.0 host or vCenter Server 5.0 using the vSphere Client
• Connecting to the ESXi 5.0 host or vCenter Server 5.0 using the vSphere Client fails
• You see the error:
  vSphere Client could not connect to “vCenter_Server_IP_Address“.
  An unknown connection error occurred. (The request failed because the client could not validate the server’s SSL certificate. (The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.))”
• This issue occurs on WIndows 2003 Server and Windows XP machines

Resolution

To resolve this issue, install/reinstall Microsoft .NET Framework 3.5 Service Pack 1 or later.

To download Microsoft .NET Framework 3.5 Service Pack 1, see the Microsoft Download Center.