[2020] An error appears

We will need to have a look at your output_log.txt (called player.log on Mac and Linux). Here's where to find the log depending on your operating system. To easily find the local files for the game, right click your game in the Steam library, select Properties, go to the Local Files tab and select Browse local files.

Windows
\SteamApps\common\Cities_Skylines\Cities_Data\output_log.txt

Mac OS X
Users//Library/Logs/Unity/Player.log

Linux
~/.config/unity3d/Colossal Order/Cities: Skylines/Player.log

As we can't attach files here on Steam, you will need to upload the file somewhere else. Here's a few sites you can upload it to, which do not require you to sign up first:

https://uploadfiles.io/ - free, no sign-up, can handle large log files
https://paste.ee/ - fast, no sign-up required (some log files will be too large for it though)
https://zerobin.net/ - bit clunky, but can handle huge files, no sign-up required

Looking at your mods, there are a few you need to unsubscribe and get the newer versions of:

- Painter - get Repaint instead
- Ploppable RICO - get Ploppable RICO Revisited instead (this is almost certainly causing the error)

Make sure to enable the new mods as they'll be disabled by default. Then restart the game and load your save. If the error is still there, upload a new log from that attempt and I'll have a look again. But it looks like the error that should be fixed in the newer RICO. So crossing fingers that'll do the trick. :)