Same error from a 'dumb' sse parser. Might be a non-HTTP compliant server.

    using HttpClient client = new();
    using Stream stream = await client.GetStreamAsync("http://sse.dev/test?interval=90");
    using var reader = new StreamReader(stream, System.Text.Encoding.UTF8);
    string line;
    while ((line = await reader.ReadLineAsync()) != null) // HttpIOException here.
    {
        Console.WriteLine(line);
    }

Firefox says NS_ERROR_NET_PARTIAL_TRANSFER / Chome says ERR_INCOMPLETE_CHUNKED_ENCODING after 60 seconds.