NullReferenceException in System.Threading.Tasks calling HttpClient.GetAsync(url)

I have a strange problem in my MVC 4.0 application. I use REST web services (Amazon Associate) . I created a method, which I use from everywhere. The shortened version is this:

    private async Task<XElement> GetRequest(string url)
    {
        string myresponse;
        HttpResponseMessage response = null;
        HttpClient client = new HttpClient();            
        try
        {
            response = await client.GetAsync(url);
            myresponse = response.Content.ToString();
            if (myresponse.Contains("503"))
            {
                myTrace.WriteLine("503 Sleep.....");
                Thread.Sleep(3000); // looks like amazon us does not like fast requests....
                return await GetRequest(url); //restart after pausing....
            }
        }
        catch (TaskCanceledException ex)
        {
            myTrace.WriteLine("TaskCancelled From GetRequest: " + ex);
            return null;
        }

        catch (HttpRequestException ex)
        {
            myTrace.WriteLine("RequestException Sleep.....");
            Thread.Sleep(300000); // 5 minutes de pause 
        }

        catch (Exception ex)
        {
            myTrace.WriteLine("From GetRequest: " + ex);
            return null;
        }

        try
        {
            XElement content = await response.Content.ReadAsAsync<XElement>();
            response.Dispose();
            client.Dispose();
            return content;
        }
        catch (Exception)
        {
            return null;
        }
    }

Nothing fancy, it does work perfectly well....But, now, on a specific call, it bombs on client.GetAsync(url). At first I suspected something in the url to be wrong, so I grabbed it from a debugger session and pasted it directly in my browser, got the expected answer...

So, nothing wrong with the URL. Made a little Unit Test, works just fine with that same specific URL...

As