cannot logout from lxde

Dear Fedora users, I have recently experienced a problem with the lxde desktop. I have updated my linux machine from FEDORA 20 to FEDORA 21 Workstation then I have installed lxde with the following command: /yum -y install @lxde-desktop/ Since then, when I try to log out by selecting logout from the task bar menu, nothing happens and my session keeps going on. If I select reboot or shutdown from the same menu, it acts normally. Previously, I was using FEDORA 20 with lxde and I never had any problems with the logout. Any help will be welcome. Thanks in advance. Regards. Giuseppe Angelini