I had the same issue with the briefing not showing after export to SP menu. Added the brieifng.html and all started working correctly.
The briefing.html file we're adding would be better named 'debrief.html'. The information it holds does relate to victory conditions.
Thanks for the info from all other contributors, it helped out a lot.
Sometimes searching forums is like playing the early text based adventure games - you spend hours trying to find the right word or phrase that helps you reach a resolution.