you shouldn't run tests on a locked desktop. Any test that requires moving the mouse or sending keystrokes requires an unlocked desktop (especially so with Silverlight tests).
Telerik tests are not the only ones affected by this Windows limitation. You will find this is a general problem with any tool that performs UI testing. Our tests against HTML based web applications frequently can work on a locked machine because most test steps don't use the mouse or keyboard. Instead they are able to inject a Click or Select events against the element in question (e.g. a button or drop down select). Unfortunately Silverlight doesn't allow us to inject these events. We have to simulate system wide mouse moves and mouse clicks instead. Some of our customers report success using a VNC type of product.
That being said I will need more specific info in order to troubleshoot the problem. Would you be able to send us a test that demonstrates the issue? Keep in mind that the test will have to run against a publicly accessible URL so that we may run it.
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