It seems like Might of Dalaran button on World map is broken, you need to disable add-on to finish World quests with this ability.

The reload itself usually fixes this. happens on occaision.

Can confirm working with mage, pally, Warlock & Demonhunter.

but again can pop up randomly and need a reload to solve.

Will try and grab the error next time it comes up, last time iirc it was AngryWorldQuests related for me