Return code 0 means issue with the service itself or there is a server side error. Can you use Firebug ( it is a plugin for Firefox ) to see if you are interrupting any service call.
Also you can enable service tracer
that will catch the actual error you are getting.
Can you share more details about the project and if you remember any changes being made to the website prior to receiving the error. Also you can submit a support ticket with login credentials for the site so we can investigate further. If you decide to provide username and password for the site, please refer to this thread in the ticket.
the Telerik team
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