Go into "Configure Your Desktop", next click on "Workspace Appearance And Behavior", click on "Desktop Search", take away the check in "Enable Nepomuk Semantic Desktop" and click on the apply button at the bottom of the screen.
To stop the service, click on the "System Administration" button, select "Startup And Shutdown", select "Service Manager", take away the check for "Nepomk Service Module" and click on the apply button at the bottom of the screen.
These steps will turn off Nepomuk, just reboot your system and see if the error has gone.