After a fresh install of Windows (and Steam, of course),
Citites:Skylines would not start. In fact, it was the Paradox Launcher which wouldn't start. The internet was not really helpful.
This Paradox Support Article outlines what you have to do to completely uninstall and reinstall the Paradox V2 Launcher. These steps can be found all over Steam and Reddit.
What the article omits, however, is one important pre-requisite to be able to run the launcher: The Microsoft VC++ Runtime in the x64 Version (on an x64 PC, at least). You can get it
here. After installing the VC++ Runtime, the launcher started working for me. Have fun!
