Last job execution ran X days ago NC 15

Nextcloud version 15.0.2:
Operating system and version Ubuntu 18.04.1:
Apache or nginx version Apache 2.4.29:
PHP version 7.2:

A few days ago, I began to notice that in the admin panel I had the Cron background job error: Last job execution ran 4 days ago.

Much earlier than this I setup the Cron job under the Web server user www-data. And it worked flawlessly until this week.

Running crontab -u www-data -l shows me:

*/15 * * * * php -f /var/www/html/nextcloud/cron.php

Cron service is running on my server and I have successfully restarted cron. This updated the counter of the last execution of the Cron job, as well as the manual execution of Cron.php script in the nextcloud folder under the Web server user www-data:

sudo -u www-data php cron.php

How to find a solution to this problem?

What about system log?

Does it find the php binary?
Normally it is better to add the whole path to php.

For more information see:

https://docs.nextcloud.com/server/15/admin_manual/configuration_server/background_jobs_configuration.html#cron

Thanks for the responding. Here is the contents of syslog for the last couple of hours. I am new to using -nix OS, so I can not interpret the lines corresponding to the work of Сron.

Feb 3 15:35:01 srv-ph1 CRON[7289]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 15:39:01 srv-ph1 CRON[7295]: (root) CMD ( [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Feb 3 15:39:32 srv-ph1 systemd[1]: Starting Clean php session files…
Feb 3 15:39:32 srv-ph1 systemd[1]: Started Clean php session files.
Feb 3 15:45:01 srv-ph1 CRON[7400]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 15:55:01 srv-ph1 CRON[7408]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:03:32 srv-ph1 systemd[1]: Started Run anacron jobs.
Feb 3 16:03:32 srv-ph1 anacron[7414]: Anacron 2.3 started on 2019-02-03
Feb 3 16:03:32 srv-ph1 anacron[7414]: Normal exit (0 jobs run)
Feb 3 16:05:01 srv-ph1 CRON[7431]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:09:01 srv-ph1 CRON[7437]: (root) CMD ( [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Feb 3 16:09:32 srv-ph1 systemd[1]: Starting Clean php session files…
Feb 3 16:09:32 srv-ph1 systemd[1]: Started Clean php session files.
Feb 3 16:14:34 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/proc_inode_cache(1954:phpsessionclean.service)”
Feb 3 16:14:34 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/dentry(1954:phpsessionclean.service)”
Feb 3 16:15:01 srv-ph1 CRON[7525]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:17:01 srv-ph1 CRON[7528]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Feb 3 16:25:01 srv-ph1 CRON[7536]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:35:01 srv-ph1 CRON[7544]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:39:01 srv-ph1 CRON[7572]: (root) CMD ( [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Feb 3 16:39:32 srv-ph1 systemd[1]: Starting Clean php session files…
Feb 3 16:39:32 srv-ph1 systemd[1]: Started Clean php session files.
Feb 3 16:44:35 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/proc_inode_cache(1928:phpsessionclean.service)”
Feb 3 16:44:35 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/dentry(1928:phpsessionclean.service)”
Feb 3 16:45:01 srv-ph1 CRON[7662]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:55:01 srv-ph1 CRON[8154]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 16:55:53 srv-ph1 systemd-resolved[659]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Feb 3 16:55:53 srv-ph1 systemd-resolved[659]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Feb 3 17:04:49 srv-ph1 systemd[1]: Started Run anacron jobs.
Feb 3 17:04:49 srv-ph1 anacron[8305]: Anacron 2.3 started on 2019-02-03
Feb 3 17:04:49 srv-ph1 anacron[8305]: Normal exit (0 jobs run)
Feb 3 17:05:01 srv-ph1 CRON[8323]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 17:09:01 srv-ph1 CRON[8424]: (root) CMD ( [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Feb 3 17:09:32 srv-ph1 systemd[1]: Starting Clean php session files…
Feb 3 17:09:32 srv-ph1 systemd[1]: Started Clean php session files.
Feb 3 17:15:01 srv-ph1 CRON[8637]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 17:17:01 srv-ph1 CRON[8640]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Feb 3 17:25:01 srv-ph1 CRON[8903]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 17:35:01 srv-ph1 CRON[9268]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 17:39:01 srv-ph1 CRON[9280]: (root) CMD ( [ -x /usr/lib/php/sessionclean ] && if [ ! -d /run/systemd/system ]; then /usr/lib/php/sessionclean; fi)
Feb 3 17:39:32 srv-ph1 systemd[1]: Starting Clean php session files…
Feb 3 17:39:32 srv-ph1 systemd[1]: Started Clean php session files.
Feb 3 17:44:39 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/proc_inode_cache(1770:phpsessionclean.service)”
Feb 3 17:44:39 srv-ph1 snapd[889]: udevmon.go:190: udev monitor observed remove event for unknown device “/sys/dentry(1770:phpsessionclean.service)”
Feb 3 17:45:01 srv-ph1 CRON[9499]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
Feb 3 17:47:21 srv-ph1 systemd[1]: Created slice User Slice of ngrin.
Feb 3 17:47:21 srv-ph1 systemd[1]: Starting User Manager for UID 1001…