This appears to be a new issue, and difficult to replicate. You may need to refresh the cache of the browser system you are using and the refresh the game. The technical team has already been advised of the problem.
In the meantime, if the refresh/clear method does not work, you should submit a support ticket and the quest can be unlocked manually.
It appears that you were able to finish this quest, but don't hesitate to contact our customer support department if you experience similar issues in the future.