My game started to crash every time I unload the world. I combed the crash log to see what might be the problem. I found 3 different issues around the time of the crash.
The first one was a sound error, I found the offending mod and looked at their page. There were no reports of crashes regarding that error, just some glitchy behavior.
The second error was regarding a hand full of vanilla textures with this error: "Failed to parse texture header!"
The third error is the only one that looks serious:
Exception occurred: System.NullReferenceException: Object reference not set to an instance of an object.
at ModularEncountersSystems.World.NpcData.Update()
at ModularEncountersSystems.World.NpcData.ProcessSecondaryAttributes()
at ModularEncountersSystems.World.NpcData.<ProcessPrimaryAttributes>b__47_0()
at Sandbox.MySandboxGame.ProcessInvoke()
at Sandbox.MySandboxGame.Update()
at Sandbox.Engine.Platform.Game.UpdateInternal()
at Sandbox.Engine.Platform.Game.RunSingleFrame()
at Sandbox.Engine.Platform.FixedLoop.<>c__DisplayClass11_0.<Run>b__0()
at Sandbox.Engine.Platform.GenericLoop.Run(VoidAction tickCallback)
at Sandbox.Engine.Platform.Game.RunLoop()
at Sandbox.MySandboxGame.Run(Boolean customRenderLoop, Action disposeSplashScreen)
at SpaceEngineers.MyProgram.Main(String[] args)

Now I haven't played since RivalAI merged with MES and my world contains several uncaptured grids from before the merge. Maybe that has something to do with it?

Looks like I have to split the log since Steam won't let post.