Schedule backup don't work

[Iled]

Hi.
After update my DS-106j to FW DSM 2.1-0844 is the Scheduled backup to my USB drive not working properly anymore.
If I make the backup manually it's OK, but on Schedual it's working sometime and sometime not.
When it's not working I will not recive a fault message in mail or in the system logg.
What could be cause of this fault?