If you have finished everyone's storylines, you should go in the castle for the final confrontation with Mantras, doesn't that work ?
Steam is the same so that shouldn't be an issue, maybe can you send me the savegame so I can check?
If you don't know how, I made some instructions here in my forums: https://www.winterwolves.net/viewtopic.php?f=33&t=2971