Bug 185419

do you get the log out dialog that lets you choose between reboot/shutdown/logout? or does no dialog appear at all when you press "Leave"?

I click on the Application Launcher (blue square icon with the letter K), then choose "Leave".
Then I see several choises: "Log out", "Switch off", "Reboot", "Suspend to RAM", something else.
I click on "Log out". The application launcher window disappears, and nothing happens.
Same thing with "switch off"

ok, so this is a ksmserver<->dbus<->plasma issue.

if you run this command from a konsole window, what do you get:
qdbus org.kde.ksmserver /KSMServer

method bool org.kde.KSMServerInterface.canShutdown()
method QString org.kde.KSMServerInterface.currentSession()
method void org.kde.KSMServerInterface.logout(int, int, int)
method void org.kde.KSMServerInterface.resumeStartup(QString)
method void org.kde.KSMServerInterface.saveCurrentSession()
method void org.kde.KSMServerInterface.saveCurrentSessionAs(QString)
method QStringList org.kde.KSMServerInterface.sessionList()
method void org.kde.KSMServerInterface.suspendStartup(QString)
method void org.kde.KSMServerInterface.wmChanged()
method QDBusVariant org.freedesktop.DBus.Properties.Get(QString interface_name, QString property_name)
method void org.freedesktop.DBus.Properties.Set(QString interface_name, QString property_name, QDBusVariant value)
method QString org.freedesktop.DBus.Introspectable.Introspect()

dbus-monitor says after clicking on inactive decktop

method call sender=:1.11 -> dest=org.freedesktop.DBus path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=AddMatch                              
   string "type='signal',path='/kickoff',interface='org.kde.plasma',member='reloadMenu'"                                                                       
method call sender=:1.11 -> dest=org.freedesktop.DBus path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=RemoveMatch                           
   string "type='signal',path='/kickoff',interface='org.kde.plasma',member='reloadMenu'"                                                                       
method call sender=:1.11 -> dest=org.freedesktop.DBus path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetNameOwner                          
   string "org.kde.ksmserver"                                                                                                                                  
method call sender=:1.11 -> dest=org.kde.ksmserver path=/MainApplication; interface=org.kde.KApplication; member=updateUserTimestamp                           
   int32 27536665                                                                                                                                              
method return sender=:1.7 -> dest=:1.11 reply_serial=178                                                                                                       
method call sender=:1.11 -> dest=org.freedesktop.DBus path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=GetNameOwner                          
   string "org.kde.ksmserver"                                                                                                                                  
method call sender=:1.11 -> dest=org.kde.ksmserver path=/KSMServer; interface=org.kde.KSMServerInterface; member=logout                                        
   int32 -1                                                                                                                                                    
   int32 0                                                                                                                                                     
   int32 -1                                                                                                                                                    
method return sender=:1.7 -> dest=:1.11 reply_serial=180

Apparently I've encountered the same or similar bug as described by OP. In KDE 4.2.0, selecting Logout choice (from Session{Logout/Lock/Switch User}/System{Suspend to Disk/Restart/Shutdown} buttons that appear after hovering mouse over Leave icon on KDE4 menu) does not result in a successful logout from that session, and session is dead thereafter: KDE4 menu does not pop up in subsequent access attempts. I've tried killing the dead session (by killing X), removing .kde and .kde4, restarting kdm, and entering a new session from there; within this new session I can Logout successfully only *once*, before the above behaviour returns in subsequent sessions. System is as follows:

GNU/Linux Gentoo kernel 2.6.28-gentoo-r2
kde-base/ksmserver-4.2.0
compiled with
gcc version 4.3.3

this bug is fixed in kde 4.2.3

I have exactly the same problem, but in KDE 4.2.98 (Ubuntu 9.04, PPA repository). When I try to log out, nothing happens. If I try to log out through pressing Ctrl+Alt+Del, the window offering logout/shutdown options simply disappears and never reappears on subsequent Ctrl+Alt+Del. When trying to log out through KMenu/Leave, everything happens the same way as in Vladimir Eremeev descrpition.

Works for me in KDE 4.9.0