jabonshampu Halfling.Serialization.DeserializeException: Deserialization from source "<[user's home folder]\Documents\My Games\Cosmoteer\Data\rules.txt>" failed.
almost every modding crash start like this, like jared said we need the whole log
for updating an old mod, you need to find the outdated parts of the code and replace it with code that work, it's possible to do, rather easy with small mods but it get complicated with bigger mods
but if the mod is really old, it's easier to understand what it was supposed to do, and code from scratch for the newest version while reusing the sprites
(and don't forget to ask the mod creator if it's ok to update the mod)