Eye of the Brandstorm: Invincible Branded Wyvern

During the Season 4 Episode 1 Story mission "Eye of the Brandstorm" the Branded Wyvern never becomes targetable or vulnerable to any form of attack. Meanwhile it is absolutely willing and able to beat the living daylights out of player characters. This issue blocks story progression since the mission can not be completed without beating the final boss.

EDIT: Third try was the charm. This mission seems to have issues recently. Rytlock and/or Canach often appear as floating weapons, and now there is this godmoding Wyvern. Never happened during the first eight playthroughs of this story episode.