I've got pretty much the same problem but all I did was enter through the top, kill a few priests and then leave to recruit another party member. When I get back, it crashes like mrmonocole describes but in my output I get:
ArgumentException: An element with the same key already exists in the dictionary.
at System.Collections.Generic.Dictionary`2[system.Guid,ObjectPersistencePacket].Add (Guid key, .ObjectPersistencePacket value) [0x00000] in <filename unknown>:0
at PersistenceManager.SaveObject (.Persistence persistence) [0x00000] in <filename unknown>:0
at PersistenceManager.SaveGame () [0x00000] in <filename unknown>:0
at GameResources.BuildSaveFile (System.String name) [0x00000] in <filename unknown>:0
at GameResources.SaveGame (System.String filename) [0x00000] in <filename unknown>:0
at GameState.Autosave () [0x00000] in <filename unknown>:0
at GameState.FinalizeLevelLoad () [0x00000] in <filename unknown>:0
at LevelStartWrapperExit.Update () [0x00000] in <filename unknown>:0
Save / DxDiag / outpu here : https://drive.google.com/file/d/0B7JWGntnoLVXU1pVS0FFU0lfc0U/view?usp=sharing