Cronjobs not working reliable since version 1.17.2.

Ever since we have upgraded the cronjob for the mail send/pull isn´t working any more.

I can see the OS running the cronjob (just an example for today):
Jan 08 17:05:01 koren-deploy-02 CRON[19447]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:10:01 koren-deploy-02 CRON[19595]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:15:01 koren-deploy-02 CRON[19604]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:20:01 koren-deploy-02 CRON[19616]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:25:01 koren-deploy-02 CRON[19627]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:30:01 koren-deploy-02 CRON[19636]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:35:01 koren-deploy-02 CRON[19645]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:40:01 koren-deploy-02 CRON[19798]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:45:01 koren-deploy-02 CRON[19806]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:50:01 koren-deploy-02 CRON[19814]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 17:55:01 koren-deploy-02 CRON[19826]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:00:01 koren-deploy-02 CRON[19835]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:05:01 koren-deploy-02 CRON[19844]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:10:01 koren-deploy-02 CRON[19991]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:15:01 koren-deploy-02 CRON[20000]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:20:01 koren-deploy-02 CRON[20060]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:25:01 koren-deploy-02 CRON[20072]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:30:02 koren-deploy-02 CRON[20081]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:35:01 koren-deploy-02 CRON[20090]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:40:01 koren-deploy-02 CRON[20236]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:45:01 koren-deploy-02 CRON[20245]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:50:01 koren-deploy-02 CRON[20254]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)
Jan 08 18:55:02 koren-deploy-02 CRON[20283]: (www-data) CMD (/usr/bin/php /var/www/support/api/cron.php)

But I there is still mail in in inbox folder for imap (office365 account). Also the logging in Osticket shows me this (wich is consistent with the last new tickets that are creates:

The problem has been going on for some time (about 6 weeks) and I have missed several important tickets. I can´t find any issue in the underlying OS and it has worked perfectly before the upgrade tot 1.17.2.

I can, temporarily fix this by running the same command via sudo as the www-data user. Afterwards it works for about a week or 2 (some times less)

What can i do to debug / solve this issue?