It seems the issue is solved. Due to a bad BIOS-Setup (dynamic clock ticks) the time slow down. NTP wasn't be able to correct this. After fixing this the observed misbehavior vanished.
The Bug can be closes. Sorry for the noise. -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected] Archive: http://lists.debian.org/[email protected]

