On some  machines, we have noticed a problem with transparencies when we restore a session. What happens is that  the displayed element ( inline ,crossline, timeslice, etc ..) can disappear from the display, or there is no display  when we move the mouse into the scene.
It can also happen when clicking for instance on the view mode, or when trying to restore an other saved session .

This is characteristic of bugs in Nvidia OpenGL drivers. It has already been reported with no reply yet.

To get the element  displayed again, you can do the following:

To get the problem definitely solved, you can just update your Nvidia card.