This is a common error when using this web service, especially in browser forms. One thing to know is that leveraging a web service in a browser form is essentially a double-hop of credentials. It's only one-hop when using the rich client.
Since the error doesn't give specifics, you have to check several things. First off, go check to see if the IIS site for the SharePoint site where the form resides is not set to Allow Anonymous Access. Another quick test would be to ensure that your machine has not had any network hiccups, so at least be sure you've done a reboot before testing.