VST instrument interface does not open

I am working on a project with two instances of Aria player as instrument tracks in Cubase Essential 5. I have been able to record and play back the recording, so the setting seems to work. However, my problem is that the “Edit VST instrument” button works only for one of the tracks, simply seems impossible to open the interface of the Aria in which the second of my sounds is loaded. Is this some sort of bug, or can the window be hidden in some way?