This is a tough problem to summarise in one line! Steps to re-produce:
- From Ubuntu Unity desktop (I've experienced this is 11.10 and 12.04), start an NX session with a remote server
- [do something in remote session (full screen)]
- In remote session, minimise the window (I do this by clicking in top-right corner/hotspot)
- [do something in local desktop for a bit], note grey '?' window icon for NX session in Unity task panel
- Try to go back to remote session (goto 2). If you can't click remote session's icon in task panel, try ALT+TAB
Now, on some occasions it will be no problem (seems that it always works first time, but not subsequently), and the window of the remote session will spring back into life. On other occasions there is only a tiny grey rectangle about 5 x 20 pixels that appears but doesn't allow me to re-activate the NX window. If this happens I need to go back to the menu and go through the steps to start a new session (thankfully however once it connects I am returned to the existing session).
Here is a screenshot showing the grey rectangle, and you can also see the grey '?' icon in the task panel, with a pop-up title of 'HUD'.
Update: I've now installed gnome-shell, and I get the same problem with that. First switch works well, but second one fails and I get the little grey rectangle.