Please make sure that:
1. The account which you are trying to use has the administrator role.
2. If this is a new account which you have created to be used by Thunder, you will need to log in with it using the browser at least once before Thunder can use it.
3. If you are using forms authentication, the first test connection might say that it fails. The second one after that should be successful.
Please let me know if this helps.
All the best,
the Telerik team
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