If you're here, I assume you already know that your game is crashing from a null pointer related to funlockfile. Something like this:
If anything is missing from this guide, please do let me know in the comments! I'm also open to fixes for other *nix OSes, so leave those down there too please!

Any stupid questions in the comments will elicit no response from me.

The game is written in C#, and on Windows/macOS the runtime is installed automatically (not sure here, don't quote me). On Linux, however, it isn't. This isn't a problem most of the time, but the moment you start using mods it appears more often.

Debian/Ubuntu (tested on Linux Mint 20.1):
Now try starting your game and trying to reproduce your crash.

This will break the Steam Overlay, so if you need it do not follow these steps!

Open up a terminal and run This should spit out a number over at least 1,000,000. If it isn't, follow some internet guide to change the hard open file limit on your system. Change it to at least 1,048,576.

Next, run This should spit out exactly one result. Open up that file in your favorite code editor and add this line directly below the line that says Save and exit.

Finally, open up game properties and set the launch location to Replace /path/to/Cities_Loader.sh with the path to Cities_Loader.sh. Try starting your game again and reproducing the crash.