scheduled posts and other cron issues

Hi there,
I don’t know why, my wordpress installation cannot publish scheduled posts, and some plugins missed their cronjobs, i.e backwpup since 3 weeks.
I even installed a new wordpress in a subfolder without any plugin enabled. but the problem persists and scheduled posts cannot be published by the time scheduled.

I contacted the hosting company, they said the cronjobs on their servers are working properly without any problem.

How can I find the problem? Is there a debug system I can look into?

wp version: 4.5.3