Edit: This was caused by an addon and can be ignored.
I was completing bonus objectives ion Hallowfall, and one of the bonus objectives printed a message that said it failed when I finished the requirements. I didn’t think much of it, and went off to the next bonus objective: The Midnight Sentry.
When I completed The Midnight Sentry, I received the same error The Midnight Sentry Failed.
When I checked my quest tracker, it showed this:
Bonus Objectives
The Midnight Senttry
✓ 10/10 Shadowed Gloomrenderer slain
✓ 1/1 Enemy revealed using Sentry Flare (Optional)
I thought it was caused by me being near the cap on Valorstones so I went to Durogal and spent some. When I came back, it’s still showing all the objective items complete, but the quest won’t complete.