Graph API - HttpClient returns 404, but curl (and Graph Explorer) for same request works?

var token =  await GetUserTokenAsync(); //gets the latest token from the GraphClient

using (var client = new HttpClient())
{
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
    using (var content = new StringContent(body))
    {
        content.Headers.ContentType = new MediaTypeHeaderValue("application/json");

        var postUrl = new UriBuilder($"https://graph.microsoft.com/v1.0/groups/{groupId}/threads/{threadId}/reply").ToString();

        var resp = await client.PostAsync(postUrl, content);

        if (!resp.IsSuccessStatusCode)
        {
            //same Url and body
            Console.WriteLine($"Error: Unable to post reply to group {groupId} thread {threadId}. The server said {await resp.Content.ReadAsStringAsync()}");
            String curlCommand = $"curl -i -X POST -H \"Authorization: Bearer {token}\" -H \"Content-Type: application/json\" -d '{body}' \"https://graph.microsoft.com/v1.0/groups/{groupId}/threads/{threadId}/reply\"";
            Console.WriteLine($"Try this with curl: {curlCommand}");
        }
    }
}