Hi Isatis,
Loading and initializing an environment is a lengthy process. It can take a very long time on some systems. Be thankful if it completes successfully.
If it never completes, or if QuestViewer reports an error and closes, first restart your computer and try to run that mission before you run any other programs.
Report the exact name of the mission on the Missions board if the problem persists. Also post your DxDiag on the Technical support board. Other posts on that board will tell you how to upload your DxDiag.
Regards,
Marty