Snapshots not running on schedule

Snapshots are not running on schedule. They work when I manually do a new snapshot but not when it's supposed to run on schedule or when I click "Run now".

The site is active and has many visitors, WP Cron should be working, there is lots of memory available, and I even have other sites on the same server that work perfectly fine.

I see there are several other threads about this problem, is it a known problem?

Please help
Shaun