UFT/QTP - Can not find Objects parent

I can not access an object in my testcase. The object is a Tab, attached to a (Sub)Window in my application. Befor and after the Testcase the Tab and the window the tab is attached to is found by the object spy fine, but while the testcase runs the Error below comes up .

Cannot find the "[ActiveX]" object's parent "[Window]" (class Window). Verify that parent properties match an object currently displayed in your application.

I access the object so : VbWindow("ApplicationWindow").Window("SubWindow").ActiveX("Tab")

I am new to UFT, so I searched a while and found some solutions but they seem not to work for me, because they always related to browser testing, not application.