Have you tried changing blocks (including changing to challenge block and back to a normal one) as well as logging out and back in?
yes, tried all of those, and it doesn't address the issue
Is the ARKS Quest still marked as incomplete, or have you not yet accepted the reward for it?
It is complete, and rewards are accepted, as seen from screenshot.
Update: for some reason around the time of this post, it is working fine suddenly, and I have no idea why.