Never heard of the problem before, but might be something to do with Defiler since it isn't up to date with the AE version of the game.  Try a character without using Defiler at all and see what happens. 

If it happens again and the game is a legit copy then report it to THQNordic here

https://redmine.thqnordic.com/projects/titan-quest-atlantis-community/

Blog post giving some info on what the devs are looking to implement/work on atm.

https://titanquestgame.com/titan-quest-the-future/

I've been on about the bad sound balance in the game's expansions for a while now.  Most of the sounds are not even needed and many seem to come from nowhere and for no good reason.  Volume is also very mixed.  I've more or less given up playing my Dragon Hunter because if I have the game volume where I normally have it I then get blasted out of my chair by Thunder Strike.  I get that the devs want it to sound like thunder, but it really doesn't need to be that far out of whack with the rest of the sounds in the game.

Welcome to the game and to the forum.    If you haven't found it yet, Grim Dawn is TQ's spiritual successor so have a look at that too.  Made by some of the developers who worked on TQ at Iron Lore it uses the TQ engine so has a lot of similarities.  That said it's a much more complex game and the engine has been reworked/upgraded a lot over the years. Setting is apocalyptic which doesn't suit everyone, but it has a lot of lore giving the background/history of the world in it.

Nice work and updated Tauceti. 

Best to report it as a bug here

https://redmine.thqnordic.com/projects/titan-quest-atlantis-community/

That's THQNordic's direct contact for these things so will go straight to the devs.

Well, if you did the quest then I guess it's fixed since the boss didn't spawn before so completion wasn't possible.