Greetings.
I have not been able to find any information on this so it might be a local issue. Several times my LoastEpoch.exe has crashed just after completing a Monolith boss and prior to the Harbinger spawning.
Has anyone else had this issue?
You should use the bug report tool in-game since that is the most helpful for the devs to fix the problem because it sends a bunch of game state data (even after a crash it is still helpful and most of the data is still there).