Hey there, I am getting the exact same error code:
at TaleWorlds.CampaignSystem.Roster.TroopRoster.AddToCountsAtIndex(Int32 index, Int32 countChange, Int32 woundedCountChange, Int32 xpChange, Boolean removeDepleted)
at TaleWorlds.CampaignSystem.Roster.TroopRoster.RemoveTroop(CharacterObject troop, Int32 numberToRemove, UniqueTroopDescriptor troopSeed, Int32 xp)
at TaleWorlds.CampaignSystem.Hero.AfterLoad()
at TaleWorlds.CampaignSystem.CampaignObjectManager.CampaignObjectType`1.AfterLoad()
at TaleWorlds.CampaignSystem.CampaignObjectManager.AfterLoad()
at TaleWorlds.CampaignSystem.Campaign.OnGameLoaded(CampaignGameStarter starter)
at TaleWorlds.CampaignSystem.Campaign.DoLoadingForGameType(GameTypeLoadingStates gameTypeLoadingState, GameTypeLoadingStates& nextState)
at TaleWorlds.Core.GameType.DoLoadingForGameType()
at SandBox.SandBoxGameManager.DoLoadingForGameManager(GameManagerLoadingSteps gameManagerLoadingStep, GameManagerLoadingSteps& nextStep)
at TaleWorlds.Core.GameManagerBase.DoLoadingForGameManager()
at TaleWorlds.MountAndBlade.GameLoadingState.OnTick(Single dt)
at TaleWorlds.Core.GameStateManager.OnTick(Single dt)
at TaleWorlds.MountAndBlade.Module.OnApplicationTick_Patch1(Module this, Single dt)
So far the only mods we are using and are overlapping are the core mods (Harmony, UIextended, ModMenu and ButterLib) and Hot Butter. That give me the assumption that either one of these mods, the main game or a combination from both is the issue.
One last question... were you able to reproduce the error in Vanilla game?