Then deleted everything in that Spotify folder and restarted the Spotify app. It made me sign back in but everything played and worked!
From what I've read the issue can be songs that are saved locally, either the file is corrupted or not communicating with the app.
PS: To get to the hidden library where these files are located (on Mac) click your desktop then click "GO" in the top menu. Once it drops down, hold the option/alt key and you will see Library appear, then navigate to the file path Library/Application Support/Spotify.
Hope this helps some people!
For Mac users, this is the correct solution.
Actually, there are 3 libraries in macOS. System, Main and User Libraries. You are speaking of the User library and that is the correct path.