After a week of investigation I have finally solved this problem. I used these commands:
$ sudo snap disable nextcloud
$ sudo chown -R root:root /media//<hard_drive_name>/nextcloud_data_directory
$ sudo chmod -R 0770 /media//<hard_drive_name>/nextcloud_data_directory
Re-enable your Nextcloud server
$ sudo snap enable nextcloud
$ sudo nextcloud.occ files:scan --all