This may be caused by a bug in an old app version. You can fix it by restoring your Locker:
1. Export all your Locker files by going to Settings > Export All Files > Choose "Copy To Files"
2. Go to your "Files" app on your phone and open the LockerExport.zip and confirm all the files you have are there and working
3. Once you've confirmed all your files are saved outside your Locker, you can delete the Locker app and re-download. (If you have Premium, go to Settings > Get Premium Pass > Restore Purchases)
4. In your Files app, share each file to the Locker app that you want in your Locker.