When I try to log out or power down my computer, the current kde session stays in the closing state.
In this case, loginctl session-status prints something similar to this:

 - olesalscheider (1000)
           Since: Sat 2016-04-23 15:56:16 CEST; 8min ago
          Leader: 1121
            Seat: seat0; vc1
         Display: :0
         Service: sddm; type x11; class user
         Desktop: KDE
           State: closing
            Unit: session-1.scope
                  �~T~\�~T~@1292 /usr/x86_64-pc-linux-gnu/bin/pulseaudio --start --log-target=syslog
                  �~T~\�~T~@1494 gpg-agent --homedir /home/olesalscheider/.gnupg --use-standard-socket --daemon
                  �~T~\�~T~@1616 kdeinit4: kio_http_cache_cleaner [kdeinit]
                  �~T~T�~T~@1827 /usr/x86_64-pc-linux-gnu/libexec/kf5/kio_http_cache_cleaner

Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: QDBusAbstractAdaptor: Cannot relay signal KDEDModule::moduleDeleted(KDEDModule*): Pointers are not supported: KDEDModule*
Apr 23 16:02:51 OleDesktop kactivitymanagerd[1231]: The X11 connection broke (error 1). Did the X11 server die?
Apr 23 16:02:51 OleDesktop kdeinit5[1191]: The X11 connection broke: I/O error (code 1)
Apr 23 16:02:52 OleDesktop drkonqi[2550]: QXcbConnection: Could not connect to display :0
Apr 23 16:02:52 OleDesktop kdeinit5[2555]: QXcbConnection: Could not connect to display :0

It seems that the problem is that the X server is terminated before kdeinit5 finishes and before it can terminate all children.
But AFAIK the X server should only be terminated once startkde finishes and that can only happen when the kdeinit5_shutdown wrapper for kdeinit returns.
Therefore it seems likely that the bug is either in kdeinit or the wrapper.

The session can be cleanly terminated by running "loginctl terminate-session".

Reproducible: Always

Steps to Reproduce:
1. Log out
2. Run loginctl list-sessions / loginctl session-status

Actual Results:  
Session stays in closing state

Expected Results:  
Session finishes cleanly