Cubase and Nuendo may freeze on start or the video engine crashes when loading projects containing video files.
An installed "Flip4Mac" (a component that adds Windows Media Video support to QuickTime) can cause this and the crashes seem to happen during the license validation.
Make sure to have a valid license available for "Flip4Mac". If that does not help, the only solution is to remove it from the system. A mid term solution from us will be an updated video engine that does not rely on QuickTime anymore.