TextInput::openSoftwareInputPanel method doesn't do anything if it's called when there is no application widget that has the keyboard focus. So on Symbian platform the application has to be already launched and in the foreground when one uses this method to open the input panel. In the example code (FocusScopeExperiment) that is attached in the original error report (https://bugreports.qt.nokia.com/browse/QTCOMPONENTS-1162) the openSoftwareInputPanel method is called in Component.onCompleted method that gets executed before the application has gained foreground and before the QApplication::focusWidget has been set (that is before QWidget::showFullScreen has been executed in FocusScopeExperiment's main) so that particular openSoftwareInputPanel method call won't do anything.
If one wants to have the input panel open e.g. when the application is launched, the most viable solution in general when taking into account different Symbian platform versions is to use a timer to open the input panel; one could start the timer e.g. when one gets the notification that the application has been activated or when the text input has gained the (active)focus.