I got the red icon too for one of the quests, think it was because I mounted up to get to where I was going, and when you summon a mount, any minions temporarily disappear and since you need the lamb minion to finish the quest, game thinks you can't complete it while mounted, but for some reason never restores the correct quest turn in icon when you hop off your mount.

Probably not the best wording, but that's what my guess is.