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Ahah, now after just waiting for a while and refreshing the page, this error is back: "Your data directory is readable by other users..."
I can't understand it
The Nextcloud data directory must have 0770 (rwxrwx---) permisions as shown in that same error page you got. What it doesn't say there is that the data directory also must be owned by nobody.users (65534.100) so Netxcloud would be able to access it.
By default Libreelec (udevil) automounts the usb storage as 777 (rwxrwxrwx) root.root but Nextcloud runs as user nobody.
The external storage is automounted with udevil and whose config file is /etc/udevil/udevil.conf that is located in the read only part of the file system and doesn't seem to be possible to override the default_options_ntfs to change the user and group it automounts the hdd with.
default_options_ntfs = nosuid, noexec, nodev, noatime, big_writes, fmask=0133, uid=$UID, gid=$GID, utf8
to:
default_options_ntfs = nosuid, noexec, nodev, noatime, big_writes, fmask=0133, uid=65534, gid=100, utf8
You could try to disable that permission check at boot in:
/storage/.kodi/userdata/addon_data/docker.linuxserver.nextcloud/config/www/nextcloud/config/config.php with:
'check_data_directory_permissions' => false,
but that won't work because of the fmask=0133 that makes all files readonly for non-root users.
Anyway I've not tried changing udevil.conf and I'm not sure if this would be enough for Nextcloud to have the whole data directory in an NTFS (or exFAT) drive since neither support unix type permisions. Also won't recommend messing with udevil config unless you know what you are doing.
The other (dirty) option to try would be to unmount and remount the device with the required permissions. A background script launched from /storage/.config/autostart.sh could periodically check if these are correct and unmount and remount when required, but again not sure if will work with NTFS or exFAT..
Alternatively you also can easily share an NTFS formatted disk in Nextcloud just sharing it via the own Libreelec samba server and adding it via the external storage app, with a similar result to using the -v /var/media/My\ Book:/My\ Book parameter in the addon's additional config page but still the users folders will be kept inside the container's original data directory, which may not be what you want.