the same thing happened to me today. i was able to complete the part of the quest to get the pen armor, but for some reason it bugged out at the step for me to learn the new ability. same as you, is listed as unaccepted, but there is nothing i can forfeit to retake that part of the quest - and there is no way in hell i am doing the entire questline over again. i submitted a ticket too, best of luck to us both.
I assume your char is not level 60. You can only accept the quest if you are level 60 or above, as it was changed last maintenance but not yet reflected in the in game tooltip.