Here are the reasons for the message, and the troubleshooting that can be tried:
For all instances, the first thing to try is a reinstall. This often helps the error go away.
You can also try removing all Local File sources, then add them again. For more steps on how to troubleshoot Local Files, check out this Spotify Answer.
Didn't help? Search for more Spotify Answers, or create a new thread and ask the Community.