Under My Ubuntu 12.04, Okular worke fine until I recently run apt-get upgrade. Now I can start Okular, but when try to open a pdf file, Okular just flashes and exits.
Here is the output when I run it from the terminal:

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$ okular 1.pdf
Got bus address:  "unix:abstract=/tmp/dbus-42dKIwE1xj,guid=c600816fe140597aecffe6cb0000003d"
Connected to accessibility bus at:  "unix:abstract=/tmp/dbus-42dKIwE1xj,guid=c600816fe140597aecffe6cb0000003d"
Registered DEC:  true
okular(27689)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
okular(27689)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
okular(27689)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
okular(27689)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
okular(27689)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
Registered event listener change listener:  true
okular: symbol lookup error: okular: undefined symbol: _ZN11KActivities16ResourceInstanceC1EmP7QObject


My Okular is installed from https://launchpad.net/~kalakris/+archive/okular

The "undefined symbol" usually means that there is a mis-match in libraries and thus a packaging issue

I would suggest either contacting the packager (as it's from a non-std ppa) or installing the Okular package that is shipped by Ubuntu

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okular: symbol lookup error: okular: undefined symbol: _ZN11KActivities16ResourceInstanceC1EmP7QObject

In this case, the specific library which is causing the problem is libkactivities - it is likely this Okular was intended to be used against a newer version of KDE than is present on your system.

Package error. This is how I fixed it.

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cd /usr/local/lib
sudo rm -r libQt*