Logitech keyboard and lightdm

A strange behaviour of lightdm. When using K260 keyboard lightdm is able to detect the passwords of the various users installed. If i change the keyboard for a KM270 (always logitech) neither the main user nor the others user passwords are detected. I am unable to connect any user ?
Back to the K260 resolved the problem.
Any idea of the problem ? Logitech ?
Move to KDM ?

(Netrunner 13.06, kde 4.10.5)

SM

Not so strange already metionned in :
https://bugs.kde.org/show_bug.cgi?id=318536

and a workaround there :
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-input-evdev/+bug/995715

Bug still present in kde 4.11 sc

SM