As with all expansions and updates user mods may prevent the game from working as it should. If you encounter such an issue please disable all your mods and try again. Should it work you know that the mods are the problem, however, we are unable to provide you a list with what mods are compatible and which mods aren't.
We strongly encourage you to check our Cities: Skylines forum for the latest information on mods as well as news and general information.
We know for a fact that the Traffic++ mod is not compatible with Snowfall! For users of this mod we recommend disabling it for new saves and only loading it in "ghost mode" for old saves that are reliant on it being activated to work.