Are you setting up the project as an HTML5 project when you start? Pages 34 and 142
of the User Guide http://www.corel.com/content/pdf/vsx5/vsx5_user_guide_en.pdf
If not, have you tried starting a new HTML5 project and importing the video you rendered?
From what I have read X5 does not support exporting as flv but instead favors WebMhttp://www.gripps.net/