What if:
- You installed Nextcloud via Docker
 - You created files or subfolders in the Nextcloud folder of a user
- This is also true if you’re restoring files and folders from a backup
 
 - You can’t see the files or subfolders in the Nextcloud web interface
 
Just run the following command, replacing nextcloud_app with the actual container name of your Nextcloud Docker install:
docker exec -u www-data nextcloud_app php occ files:scan --all
This is working for me, on Nextcloud version 27.0.1.