Since I was also having difficulty turning in this quest, I wanted to document the steps needed to open it up. I had finished all the quests in Drustar. Only House Waycrest remained.
Taelia was not present at all in the harbormaster's office. Flynn Fairwind was present and had the quest An Explosive Entrance. After finishing up Sweete's Strongbox and accepting Tol Dagor: The Fourth Key, both Flynn and Taelia were in the harbormaster's office. Taelia was still now accepting my quest turn in. After finishing Tol Dagor, she still would not accept the quest.
Will update after more progress is completed. The next quest Flynn offered was The Mysterious Island.