Bug with Third Boss in 'The Megalith of Phaaryl' – Can't Progress

"I'm stuck in the map 'The Megalith of Phaaryl', specifically while fighting the third boss. It has appeared, but I can't attack it and it doesn't attack me either. I tried going back to the 'Character Selection' menu, but the issue persists. I also tried closing the game and logging back in, but I'm still experiencing the same problem. Thank you."