I'VE A WARNING MESSAGE WHEN I TRY TO LOAD A CITY. WHAT DOES THIS MEAN?
This occurs when you've removed a Workshop item of your subscription list or you've a new computer.
The game remember which subscribed items you had when you build the city.
So there are three options:
1. Cancel the loading.
2. Subscribe the missing items. By selecting this option, it will subscribe all missing items at once. You may restart the game to take in account.
3. Continue the loading at your own risk. You may crash but in some cases there is no problem, for example you've subscribed an item that just change texture.