Not sure if you figured this out yet, but I can confirm that your code is working 100% when I substitute my credentials instead.
If you are sure your credentials are correct, then make sure you have the newest .net sdk installed and try again - just tried it myself with version 2.0.15.1 available here:
http://sdk-for-net.amazonwebservices.com/latest/AWSToolsAndSDKForNet.msi
and it works as expected.