Seems like after the latest patch (2.05 I believe), quests with a duty can't be completed and proceeded anymore.

Experienced this with The Lord Of Inferno (Lvl 20 main quest) and The Threat of Superiority (Lvl 15 Thaumaturge quest).

On both times I have completed the duty, but it doesn't proceed to the next step and still wants me to repeat that duty.