Make sure video playback window is open and then try to close project or Cubase and I get a “serious problem has occured” error. I then have to force quit Cubase using task manager which eventually kills the window but leaves the exe running.
If I do the same without the video window open, everything is fine.
What kind of video (codec) do you have in your project?
Do you have the latest quicktime version?
Do you have the latest version of your graphic cards driver?