SendGrid throws "Error while copying content to a stream"

I'm creating a Core 2.1 solution in Visual Studio 2017 where I send emails via Sendgrid. When trying to send an email via SendGrid, I get the following error:

An unhandled exception occurred while processing the request.

IOException: The server returned an invalid or unrecognized response. System.Net.Http.HttpConnection.FillAsync() HttpRequestException: Error while copying content to a stream.

System.Net.Http.HttpContent.LoadIntoBufferAsyncCore(Task serializeToStreamTask, MemoryStream tempBuffer)

Here is what my code looks like. I'm entering the following in parameters:

recipients: list containing "****@gmail.com"

Subject: "Hello"

Body: Html generated via Heml. It compiles in an online editor without problem.

private async Task<bool> SendAsync(List<string> recipients, string subject, string body)
{
    var client = new SendGridClient(this.configuration["Sendgrid:ApiKey"]);  
    var from = new EmailAddress( 
                       this.configuration["Administration:MainEmailAddress"],
                       this.configuration["Administration:MainEmailName"]);  
    var tos = await GetRecipientsForEnvironment(recipients);  
    var message = MailHelper.CreateSingleEmailToMultipleRecipients(
                                                          from, 
                                                          tos, 
                                                          subject, 
                                                          "", 
                                                          body, 
                                                          false);  
    var response = await client.SendEmailAsync(message);  
    return response.StatusCode == HttpStatusCode.Accepted;
}

What is the cause of this error?