I've had persistent problems with a 5 minute hang at shutdown for the last three versions of Kubuntu (13.04, 13.10, and now 14.04). I suspect this is not a KDE-specific problem. Recently, I performed a fresh installation of Kubuntu 14.04 from a live-USB, and shutdown worked normally for about a week. The hang-up is now happening again, and I can't figure out why.

A brief description of the problem:

The hang-up occurs with all methods of initiating a normal shutdown:

Clicking the shutdown or restart button in KDE,

sudo shutdown -h now,

sudo reboot

The shutdown splash screen appears. Using the down-arrow to access verbose messages, I see "Asking all remaining processes to terminate." This message remains for 5 minutes with no disk activity. Finally, a rapid series of messages flurries to the screen:

 * All processes ended within 300 seconds...                    [ OK ]
nm-dispatcher.action: Caught signal 15, shutting down...
ModemManager[852]: <warn> Could not acquire the 'org.freedesktop.ModemManager1' service name

ModemManager[852]: <info>  ModemManager is shut down

 * Deactivating swap...                                         [ OK ]
 * Unmounting local filesystems...                              [ OK ]
 * Will now restart`

Possible Sources of the Problem:

Before the problem re-appeared, I have mainly been doing routine computing. I have kept the system up-to-date using apt-get upgrade and apt-get dist-upgrade. The only other notable incident was a power failure. I do not have the computer connected to a UPS, so the power failure resulted in an immediate shutdown. Could this have corrupted an important file which must be accessed at shutdown? Is there any way that could cause a 5-minute hang-up?

Here is a list of packages that have been updated before the problem appeared:

bash iotop dpkg dpkg-dev python3-software-properties libdpkg-perl software-properties-kde software-properties-common akonadi-backend-mysql libakonadiprotocolinternals1 akonadi-server firefox-locale-en firefox flashplugin-installer libqapt2 libqapt2-runtime thunderbird openjdk-7-jre-headless thunderbird-locale-en kubuntu-driver-manager qapt-deb-installer openjdk-7-jre qapt-batch icedtea-7-jre-jamvm libelf1 dpkg dpkg-dev libdpkg-perl libjbig0 gettext-base libgettextpo-dev libssl1.0.0 libgettextpo0 libasprintf-dev linux-headers-3.13.0-24 gettext libasprintf0c2 linux-headers-3.13.0-24-generic openssl linux-libc-dev gstreamer0.10-qapt kubuntu-desktop linux-image-extra-3.13.0-24-generic linux-image-3.13.0-24-generic

I would appreciate any help with this.