[horde] Memcached sessions don't expire after upgrade to 5.1.0

[OVi C]

Hi.
After upgrading to latest horde groupware webmail version 5.1.0 from 5.0.5,
there seems to be a lot of sessions opened from the last days.  Also using
the command line horde-active-session it shows that there are active
sessions, but there is nobody logged in. Even when I close the browser the
session aren't killed. I'm using memcached to store the session
information.  Prior to upgrade the sessions would expire gracefully, now it
seems to add up every time users log in.
They only go away if I restart the memcached service.

Any hints ?