"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

For some reason, the barrier won't open for me, even though all the heroes are standing between the demonic pillars. What could be the problem?

Are you sure you're playing version 3.1? In version 3.0, there's a bug that occurs when you perform the ritual with a hero other than Arantir.