If you have started the quest Stifled Light and progressed to the Dunskeig Sewers leg of the questline, you will be unable to accept Sentinel Fall. There are other repeatable quests throughout the questline that provide greater xp rewards, though perhaps not in as rapid a fashion as Sentinel Fall did.
The amount of xp that Sentinel Fall awarded was much too great vs the ease to complete the quest. The numbers have been reduced slightly, and the repeatable quests at the end of the new stretch of the questline (Twistier Crystals and More Weeding Out The Weak) have received a boost to their xp in the most recent update.
Going forward we'll likely offer an increased xp reward on the repeatable quests at the end of any given leg of the continuing questline while it is "current content", and reduce that xp down to a more reasonable level when more quests are added beyond.
Ranger - 172 - Balor - Android