Immediately as I finished one of the optional area of the Flesh Forest (a cave in which I had to kill a specific monster iirc), the opening cutscene for act 2 started and the questline was restarted, allowing me to pursue Valera all over again. I didn't check if I could still gain XP from the quests or what would happen if I caught up with my current progression. I will test it as soon as I can.