Cronjob "ncc files:scan --all" not working

You mean when you upload/change files via the official Nextcloud Windows client, those changes are not visible in official Android and iOS apps?<

Yes, exactly. I wasn’t able to get data visible, when deploying it via nextcloud-windowsapp. Journal of trial/error in this thread:
https://dietpi.com/forum/t/nextcloud-folder-not-visible-in-dietpi-and-aria2/14820/41

Are uploads/changes done via Windows client visible on the server e.g. when accessing the web interface?

Yes, as long, as i create folders within Nextcloud (Apps or WebUI)

For the cron job, just to check whether they run now successfully:
journalctl -u cron

The output of the last 5 minutes:

Nov 05 13:50:02 DietPi CRON[22453]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:50:02 DietPi CRON[22451]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:50:02 DietPi CRON[22451]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:51:01 DietPi CRON[22476]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:51:01 DietPi CRON[22475]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:51:01 DietPi CRON[22477]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:51:01 DietPi CRON[22478]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:51:01 DietPi CRON[22476]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:51:02 DietPi CRON[22475]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:51:02 DietPi CRON[22475]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:52:01 DietPi CRON[22493]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:52:01 DietPi CRON[22492]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:52:01 DietPi CRON[22495]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:52:01 DietPi CRON[22494]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:52:01 DietPi CRON[22493]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:52:02 DietPi CRON[22492]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:52:02 DietPi CRON[22492]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:53:01 DietPi CRON[22708]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:53:01 DietPi CRON[22707]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:53:01 DietPi CRON[22709]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:53:01 DietPi CRON[22710]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:53:01 DietPi CRON[22708]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:53:02 DietPi CRON[22707]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:53:02 DietPi CRON[22707]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:54:01 DietPi CRON[22726]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:54:01 DietPi CRON[22727]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:54:01 DietPi CRON[22729]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:54:01 DietPi CRON[22728]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:54:01 DietPi CRON[22727]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:54:02 DietPi CRON[22726]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:54:02 DietPi CRON[22726]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:55:01 DietPi CRON[22812]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:55:01 DietPi CRON[22813]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:55:01 DietPi CRON[22811]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:55:01 DietPi CRON[22815]: (www-data) CMD (php /var/www/nextcloud/cron.php)
Nov 05 13:55:01 DietPi CRON[22816]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:55:01 DietPi CRON[22814]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:55:01 DietPi CRON[22812]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:55:02 DietPi CRON[22813]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:55:03 DietPi CRON[22811]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:55:03 DietPi CRON[22811]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:56:01 DietPi CRON[22841]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:56:01 DietPi CRON[22840]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:56:01 DietPi CRON[22842]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:56:01 DietPi CRON[22843]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:56:01 DietPi CRON[22841]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:56:02 DietPi CRON[22840]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:56:02 DietPi CRON[22840]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:57:01 DietPi CRON[22862]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:57:01 DietPi CRON[22861]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:57:01 DietPi CRON[22863]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:57:01 DietPi CRON[22864]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:57:01 DietPi CRON[22862]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:57:02 DietPi CRON[22861]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:57:02 DietPi CRON[22861]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:58:01 DietPi CRON[23077]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:58:01 DietPi CRON[23076]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:58:01 DietPi CRON[23078]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:58:01 DietPi CRON[23079]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:58:01 DietPi CRON[23077]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:58:02 DietPi CRON[23076]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:58:02 DietPi CRON[23076]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:59:01 DietPi CRON[23097]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:59:01 DietPi CRON[23098]: pam_unix(cron:session): session opened for user www-data(uid=33) by (uid=0)
Nov 05 13:59:01 DietPi CRON[23099]: (www-data) CMD (echo $(date) > /tmp/test-wwwusr)
Nov 05 13:59:01 DietPi CRON[23100]: (www-data) CMD (php /var/www/nextcloud/occ files:scan --all)
Nov 05 13:59:01 DietPi CRON[23098]: pam_unix(cron:session): session closed for user www-data
Nov 05 13:59:02 DietPi CRON[23097]: (CRON) info (No MTA installed, discarding output)
Nov 05 13:59:02 DietPi CRON[23097]: pam_unix(cron:session): session closed for user www-data

Do you still see (CRON) info (No MTA installed, discarding output) output for recent calls (which would indicate an error ourput)?

Yes. In Nov 05 13:58:02 DietPi CRON[23076]: for example

I’m happy with DietPi: Pihole, PiVPN, Aria2 and Nextcloud running on one device is stunning!