Reboot Windows.
This is the only solution that worked for me after trying rebuild, delete contents of bin and rebuild, restart Visual Studio.
It's yet another example of how terrible C#/.NET build tools are.
I think (after reading many of the answers), the overall conclusion is that the cause and solution of this problem heavily depends on the setup and project, so if one answer does not work, just try another. Try non-intrusive/destructive solutions, such as restarting Visual Studio, rebooting, rebuilding, etc., FIRST, before messing with NuGet packages or reinstalling development tools. Good luck!
(NOTE: Using Visual Studio 2019, and project file was originally created in Visual Studio 2015. Maybe this helps someone investigate the issue)
(EDIT: Could this be caused by not rebooting after installing/modifying the Visual Studio installation or updating Visual Studio when the installer prompts to reboot?)