Boss: Slayer of the Arena Quest Bug

Hi,

I am playing with my Season Character and I am unable to complete the quest said in the title. It is bugged.
Here are the screenshots.
As you can see from the screenshot, the prerequisite of the quest "The One to Stop the Slayer" has been completed. However, the "Boss: Slayer of the Arena Quest Bug" quest cannot be accepted.

I've tried switching servers, using escape, disconnecting, and reconnecting however it did not resolve the issue.
The "The One to Stop the Slayer" cannot be accepted again.

Thank you.

EDIT: Picture add option does not work.