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It's a good idea to try 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.
Also, check out this help page for further tips on how to manage your local files. Here you’ll find useful info on format requirements and permitted file sources.