Unable to unlock dungeon after Patch 5.4 [The Drowned City of Skalla]

Date & Time: 12/08/2020
Frequency: Permanent after 5.4
World name: N/A
Character name: N/A
Retainer name: N/A
NPC name: Lyse
Monster name: N/A
Class/Level: Level 70
Party or solo: N/A
In-game time: N/A
Area and coordinates: Rhalgr's Reach (X:14.6, Y:9.4)
Housing: N/A
Steps:
1. After the completion of the main quest for 5.4 and the disappearance of the quest giver, the optional quest "Arenvald's Adventure" is no longer obtainable.
2. The dungeon unlocked from the quest "The Mad King's Trove", which follows Arenvald's Adventure cannot be unlocked as a result.


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I think you are confused. The Drowned City of Skalla is part of the main scenario. You cannot get to the 5.4 quests without having unlocked it, so you have unlocked it.