I have been experiencing this issue over the last couple months where BackWPup is reseting the next run to December 31, 1969 at 7:00 pm by WP-Cron.
I have tried to reset the schedule and it seems to work for a few days and then does this again. The biggest problem is that there is no warning or email that the schedule has changed and that it is no longer backing up.
The only fix that I have found is making a copy of the job or creating a new job, but again that only last a couple hours to day and it happens again.
This mainly happens only on my hourly back schedule vs. my daily schedule backup job.