First up you can try restarting your network device (router or modem), then log out and log back in to the Spotify app. It's also a good idea to check if you observe the same when connecting to a different network. If it works with another connection, it's best to contact the service provider of the original network for more information.
Another thing you can try is changing the source location of the local files on your PC. Simply create a new folder in a different directory on your device and move your local files to this folder. Then just delete the old folder used for the local files. Now you can try importing the files to your mobile device again.