I have the same issue now, I have a service account running C2WTS and it is member of WSS_WPG group which is listed in c2wtshost.exe.config file, still I am getting the same error? Anyone found a solution for this? I have used the same app-pool identity for C2WTS service. I get the same error:
No windows identity for apac\user1.SPSecurityContext.WindowsIdentity: Could not retrieve a valid windows identity for NTName='apac\user1', UPN='user1@apac.com'. UPN is required when Kerberos constrained delegation is used. Exception: System.ArgumentException: Token cannot be zero. at System.Security.Principal.WindowsIdentity.CreateFromToken(IntPtr userToken) at System.Security.Principal.WindowsIdentity..ctor(IntPtr userToken, String authType, Int32 isAuthenticated) at System.Security.Principal.WindowsIdentity..ctor(IntPtr userToken) at Microsoft.IdentityModel.WindowsTokenService.S4UClient.CallService(Func`2 contractOperation) at Microsoft.SharePoint.SPSecurityContext.GetWindowsIdentity().
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