Backup Job next start time wrong

Hello Together,

we have problem with one backup job, that is not backuping on the right time.
It is a custom backup scheme, but also appears with a simple one.

Actually it is set to do a incremental backup on Monday - Thursday every week and on every Friday a full backup. Before and after there some scripts to stop services before and shutdown the pc after the backup.
There is no error in the logging. Everything is fine. But when you click on the "Backup plans and task" overview, you see that the next start time is set to somedays later. Sometimes 2-5 days, but also last time 2 weeks later. And this is the time, the job really will start.
When you modify the job and save it, the time will be adapted to the definied time. Really strange..

We created this job allready new, but it also doesn't help.
Before we had a GFS scheme, also the same.

Do somebody have a solution statement for this problem?

Thanks in advance.
Benny