User blog comment:The Milkman/1.8 Title Update (English) Now Available on Xbox LIVE/@comment-5185059-20121125051856/@comment-4976111-20121125151236

Try reloading before the fight with Alduin and send her home. Then, try to fight Alduin on your own (or with other help, just not Serana), and then go and do the quest with Serana.