Just now I went to go host a server, and then once it starts the "Loading mission" part, this appears:
I honestly don't know what any of that means, but its not letting me host a server. I tried changing the map to see if that helped but I still got the same result.
My next guess is that its an addon, but that doesn't seem like the kind of error that an addon would cause.
I have no clue as to whats causing this so it'd be great if someone could help.