Could you please ensure the following step properties are set:
IsUrlPartial - checked
PopupUrl - contains only that part of the url that stays the same every time the test is executed
IsModalPopup - checked
ModalPopupPartialCaption - contains only that part of the dialog's caption that stays the same every time the test is executed
I hope this will solve the issue. Please do not hesitate to contact us in case you still experience problems.
the Telerik team