That empty app window at the top left of the Desktop is being reported by many people at the moment, most likely it is some app that is set to startup with Windows that is causing the problem.
Open Task Manager (Ctrl + Shift + Esc).
Select the processes tab.
Try to find the app/process in the list that starts with Windows, then right click it and choose End Task.
If you find the app causing the problem, it would be best to reinstall the app.