Update!
Also happened with the quest "Rather Be Fishin'" in Zangarmarsh with the error:
"Could not create current in quest bot; exception was thrown
Object reference not set to an instance of an object."
As before, once I manually completed the quest, I simply pressed start and it began questing again. I did not have to turn it in for functionality to return.
Edit for update: It appears to be happening to quite a few more quests, I'm not sure what the cause is tbh, it happens on all different types of quests (use item, kill x amount of mobs, etc).