Sendgrid System.ArgumentException: Unknown element: html

I just updated the following packages to their latest version:

a) SendGrid.SmtpApi updated to 1.3.1

b) Update SendGrid to version 6.0

and suddenly the WebTransport.Deliver method no longer exists - no problem, I switched to the DeliverAsync method, but now I am getting a very strange error that it has to fix from 2 years ago:

System.ArgumentException: Unknown element: html

This is the part of the stack trace that might be of interest:

System.ArgumentException: Unknown element: html

at SendGrid.ErrorChecker.CheckForErrors (HttpResponseMessage Response, Stream Stream)

at SendGrid.ErrorChecker.d__0.MoveNext ()

--- End of the stack trace from the previous location where the exception was thrown -

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (Task Task)

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (Task Task)

at SendGrid.Web.d__0.MoveNext ()

--- End of the stack trace from the previous location where the exception was thrown - to System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (Task Task)

And this is how the email sending code looks like:

SendGridMessage emailMessage = new SendGridMessage();
emailMessage.AddTo(user.CompleteRegistrationModel.UserEmailAddress);
emailMessage.From = new MailAddress(ConfigurationHelper.EmailSenderAddress, ConfigurationHelper.EmailSenderName);
emailMessage.EnableTemplateEngine(ConfigurationHelper.ConfirmationEmailTemplateId);
emailMessage.AddSubstitution("-urlaplicatie-",
                new List<string>()
                {
                    String.Format("{2}{0}{1}", user.CompleteRegistrationModel.CompanyUrlPrefix,
                        ConfigurationHelper.DomainSuffix, ConfigurationHelper.ApplicationAccessProtocol)
                });
            emailMessage.AddSubstitution("-username-",
                new List<string>() {user.CompleteRegistrationModel.UserEmailAddress});
            emailMessage.AddSubstitution("-confirmationurl-",
                new List<string>() {user.CompleteRegistrationModel.UserEmailAddressConfirmationCompleteUrl});

            emailMessage.Subject = String.Format("{0}{1}", user.CompleteRegistrationModel.CompanyUrlPrefix,
                ConfigurationHelper.DomainSuffix);
            emailMessage.Text = " ";
            emailMessage.Html = " ";

            Web webTransport =
                new Web(new NetworkCredential(ConfigurationHelper.SendgridUsername,
                    ConfigurationHelper.SendgridPassword));

            try
            {
                await webTransport.DeliverAsync(emailMessage);
            }
            catch (InvalidApiRequestException exception)
            {
                Elmah.ErrorSignal.FromCurrentContext().Raise(exception);
            }

      

        
        
        
      

    

Can someone please tell me what is going on?

Thanks, Evdin