Scheduled scan breaks for unknown reasons, Sophos logging failed

Hi,

we are in the process of moving scheduled scans to night hours.

Last night we had quite some machines which did not log the end of the scan as usual in the Scanjob.txt file:

Jobname: Nachts

Content of file Nachts.txt:

20120531 194237    Scan 'Nachts' gestartet.

Content of file SAV_20120531.txt :

20120531 194237    Scan 'Nachts' gestartet.
20120531 194950    Benutzer (NT-AUTORITÄT\SYSTEM) hat den On-Access-Scan auf diesem Computer abgebrochen.
20120531 195103    Die Erkennungsdatenversion 4.78G (Detection Engine 3.31.20) wird verwendet. Diese Version kann 3664173 Objekte erkennen.
20120531 195103    Benutzer (NT-AUTORITÄT\LOKALER DIENST) hat den On-Access-Scan auf diesem Computer gestartet.
20120531 195116    Die Erkennungsdatenversion 4.78G (Detection Engine 3.31.20) wird verwendet. Diese Version kann 3664173 Objekte erkennen.

and so on

When I go to such a machine I see that the scheduled scan does not run anymore, but why is the end of the scan not in the logs? This log entry does trigger the shutdown process of the machine here, and usually it works as expected.