When you set a backup location in the Backup Screen, OnSong remembers folder structure where you want to save the backup for that specific export location. If you receive a 404 file not found error when exporting a backup to Google Drive, it's likely because the destination folder has been renamed or deleted.
To correct this issue, open the Utilities Menu by tapping on the gear icon in the Menubar and choose Backup. Tap on Google Drive in the Export Location field. Locate a new destination for your backup, keeping in mind you may need to sign out and back into Google Drive if the issue is due to a change in permissions. Once selected, tap Done until you return to the Backup Screen. Tap on the Backup button in the upper right corner to export a backup to test the changes.