Owncloud not launching cron task

I face a strange behaviour of owncloud that i haven't been able to explain even with some research over internet.
I have setup an owncloud server on a raspberry everything is working fine except cron job functionnality.

I noticed yesterday that the timer regarding cron job say that no task were run since 2 days (probably last restart of raspberry)

After some test, I'm able to say that if I set webcron and run task via url the task goes correctly and the timer is updated. If i set cron via system and run command line task is done and timer updated. But for both if I just wait the timer keep increasing ie no 15min cycle.

Does anyone have already faced this problem or have any idea of where it can come from ?

Pretty well explained in the official documentation: https://doc.owncloud.org/server/10.0/admin_manual/configuration/server/background_jobs_configuration.html