I tried with both main heroes, but it still says I need 20 druids, and they won't upgrade for me. Maybe it's because I'm completing this quest too late, as I've progressed a lot and the script isn't working. It's a shame, but I still want to finish the game with this hero.
Added 8 minutes later
Ah, I understand what the problem is. There's a hard dependency in the script, not only on 20 druids, but also on one quest. Did you take the quest from the Elder Druids near Sheller? Because it's absolutely necessary for the ents to join.
