Hello Chao,
I am sorry to hear you are experiencing this issue. Cold you please delete and re-add the test as step and give it a try. I have also tried to reproduce this behavior but I was not able to. Another way we can proceed in order to investigate further is to provide me with the execution log so I can check the exception thrown. If the application you are testing is public available the best way to proceed is to send the test over.
Looking forward to hear from you.
Kind regards,
Ivaylo
the Telerik team