Quest not on quest list and cannot be abandoned

Hi,
I've picked up the quest 'Where Dragons Fell' from Matthias Lehner at Sindragosa's Fall in Icecrown. I came back the next day to resume it but I don't seem to be able to complete it nor abandon it.

The question mark is over his head. When I click the NPC I cannot abandon it.
The quest is not in my list of quests and nothing appears on the map.
The objectives to collect items does not appear in the tooltip either.

I really would like to complete this so it will count towards 'The Final Goal' achievement.

Any ideas?

EDIT: resolved; I disabled all addons and it reappeared in the list.