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I am experiencing the exact same issue. I was hoping that being logged out for several hours would fix the issue but it has not. I submitted a bug report as well as a ticket to customer support but have not received a response yet.
As this is the final quest for the zone it would be nice to complete it in order to have a nice transition to the next questing zone.
I spoke with one GM that provided the answer to this issue so I wanted to post for future reference. Disabling addons, /reloadui, logging out and waiting, abandoning the quest and getting again...None of that helped.
The GM suggested that I may have a malicious debuff stuck on my character and that dying may fix the issue. I let two bandits beat me to death. After respawning I was able to use the portal to Sovngarde and complete the quest.