Quote:
url: '/BackOfficeOperations/HMS/SimpleHMS.aspx.cs/checkUsername'
That's not the URL of your page. That's the URL of the code-behind file, which cannot be accessed remotely, and might not even exist if you've compiled your application.
Try specifying the correct URL for your page, by removing the ".cs":
url: '/BackOfficeOperations/HMS/SimpleHMS.aspx/checkUsername'