Finally, I was able to make a copy of the .xsession-errors file (a restart overwrote it).
here it its content:
startkde: Starting up...
kdeinit4: preparing to launch /usr/lib/libkdeinit4_klauncher.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kded4.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kbuildsycoca4.so
kbuildsycoca4 running...
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kbuildsycoca4.so
kbuildsycoca4 running...
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kconf_update.so
Invalid D-BUS member name 'idle-hint' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'is-local' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'x11-display-device' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'x11-display' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'display-device' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'remote-host-name' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'session-type' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
Invalid D-BUS member name 'unix-user' found in interface 'org.freedesktop.ConsoleKit.Session' while parsing introspection
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kcminit_startup.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_ksmserver.so
<unknown program name>(3363)/ KStartupInfo::createNewStartupId: creating: "linux-upbm;1255456632;245700;3363_TIME0" : "unnamed app"
kephald starting up
XRANDR error base: 186
RRInput mask is set!!
RandRScreen::loadSettings - adding mode: 429 2560 x 1024
RandRScreen::loadSettings - adding mode: 434 1920 x 1024
RandRScreen::loadSettings - adding crtc: 427
RandRScreen::loadSettings - adding output: 428
Setting CRTC 427 on output "default" (previous 0 )
CRTC outputs: (428)
Output name: "default"
Output refresh rate: 50
Output rect: QRect(0,0 2560x1024)
Output rotation: 1
XRandROutputs::init
added output 428
adding an output 0 with geom: QRect(1280,0 1280x1024)
adding an output 1 with geom: QRect(0,0 1280x1024)
adding a disconnected output 2
adding a disconnected output 3
output: "SCREEN-0" QRect(1280,0 1280x1024) 0 true false
output: "SCREEN-1" QRect(0,0 1280x1024) 40 true true
output: "SCREEN-2" QRect(0,0 0x0) -1 false false
output: "SCREEN-3" QRect(0,0 0x0) 0 false false
load xml
connected: 2
looking for current "SCREEN-0"
known "*" has score: 0.125
known "*" has score: 0.125
looking for current "SCREEN-1"
known "*" has score: 0.125
screen: 0 QRect(1280,0 1280x1024)
screen: 1 QRect(0,0 1280x1024)
looking for a matching configuration...
connected: 2
looking for current "SCREEN-0"
known "*" has score: 0.125
known "*" has score: 0.125
looking for current "SCREEN-1"
known "*" has score: 0.125
found outputs, known: false
activate external configuration!!
registered the service: true
screens registered on the bus: true
outputs registered on the bus: true
configurations registered on the bus: true
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kglobalaccel.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_plasma-desktop.so
kdeinit4: preparing to launch /usr/bin/knotify4
QDBusObjectPath: invalid path ""
QLayout: Attempting to add QLayout "" to QWidget "", which already has a layout
QGraphicsGridLayout::itemAt: invalid row, column 0, 0
QGraphicsGridLayout::itemAt: invalid row, column 0, 1
QGraphicsGridLayout::itemAt: invalid row, column 0, 2
Object::connect: No such signal SystemTray::Manager::jobStateChanged(SystemTray::Job*)
Invalid D-BUS interface name 'org.kde.plasma-desktop.PlasmaApp' found while parsing introspection
kdeinit4: preparing to launch /usr/lib/kde4/kio_desktop.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_trash.so
QGraphicsGridLayout::itemAt: invalid row, column 0, 0
QGraphicsGridLayout::itemAt: invalid row, column 1, 0
QGraphicsGridLayout::itemAt: invalid row, column 0, 1
Kross: "Loading the interpreter library for qtscript"
Kross: "Successfully loaded Interpreter instance from library."
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_file.so
QGraphicsGridLayout::itemAt: invalid row, column 0, 0
QGraphicsGridLayout::itemAt: invalid row, column 1, 0
QGraphicsGridLayout::itemAt: invalid row, column 0, 1
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kio_http_cache_cleaner.so
QGraphicsGridLayout::itemAt: invalid row, column 0, 0
QGraphicsGridLayout::itemAt: invalid row, column 1, 0
QGraphicsGridLayout::itemAt: invalid row, column 0, 1
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kaccess.so
<unknown program name>(3868)/ kdemain: Xlib XKB extension major= 1 minor= 0
kdeinit4: preparing to launch /usr/lib/libkdeinit4_nepomukserver.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kmixctrl.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_krunner.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_nepomukserver.so
Nepomuk server already running.
[/usr/bin/nepomukservicestub] (Soprano::Redland::BackendPlugin) creating model of type "hashes" with options "hash-type='bdb',contexts='yes',dir='/home/user/.kde4/share/apps/nepomuk/repository/main/data/redland'"
[/usr/bin/nepomukservicestub] (ServerCore) new socket connection.
(ServerCore) new socket connection.
[/usr/bin/nepomukservicestub] (ServerCore) new socket connection.
(ServerCore) new socket connection.
Application '/usr/bin/nepomukservicestub' exited normally...
[/usr/bin/nepomukservicestub] Removing connection
[/usr/bin/nepomukservicestub] (ServerCore) new socket connection.
kdeinit4: preparing to launch /usr/lib/kde4/kio_trash.so
kdeinit4: preparing to launch /usr/bin/nepomukservicestub
kdeinit4: preparing to launch /usr/bin/nepomukservicestub
Unknown service name: 10146103e3e5000125512490700000121950007_1255129065_668401
kdeinit4: preparing to launch /usr/bin/nepomukservicestub
Unknown service name: 10146103e3e5000125512490800000121950008_1255129065_668560
kdeinit4: preparing to launch /usr/bin/nepomukservicestub
Unknown service name: 10146103e3e5000125512490800000121950009_1255129065_668567
kdeinit4: preparing to launch /usr/lib/libkdeinit4_ksysguard.so
Unknown service name: 10146103e3e5000125512490800000121950010_1255129065_668810
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kmix.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_konsole.so
kdeinit4: preparing to launch /usr/bin/dolphin
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kcalc.so
kdeinit4: preparing to launch /opt/kde3/bin/kde3
[/usr/bin/nepomukservicestub] (ServerCore) new socket connection.
kdeinit4: preparing to launch /usr/bin/start-pulseaudio-x11
kdeinit4: preparing to launch /opt/kde3/bin/appspreloader.sh
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kmix.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_klipper.so
kdeinit4: preparing to launch /opt/kde3/bin/knetworkmanager
kdeinit4: preparing to launch /opt/kde3/bin/profile_chooser
kdeinit4: preparing to launch /usr/lib/kde4/kio_trash.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_file.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_file.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_file.so
W: main.c: High-priority scheduling enabled in configuration but not allowed by policy.
W: core-util.c: setpriority(): Permission denied
W: ltdl-bind-now.c: Failed to find original dlopen loader.
kbuildsycoca running...
virtual bool Phonon::Gstreamer::AudioOutput::setOutputDevice(const Phonon::AudioOutputDevice&) "default"
QLayout "unnamed" added to QVBox "unnamed", which already has a layout
QLayout: Adding KToolBar/mainToolBar (child of QVBox/unnamed) to layout for PlaylistWindow/PlaylistWindow
QObject::connect: Incompatible sender/receiver arguments
StarManager::ratingsColorsChanged() --> ContextBrowser::ratingOrScoreOrLabelsChanged(const QString&)
STARTUP
kdeinit4: preparing to launch /usr/bin/skype
kdeinit4: preparing to launch /usr/bin/firefox
*** nss-shared-helper: Shared database disabled (set NSS_USE_SHARED_DB to enable).
Currently in use: 3 blocks containing 72.2 KiB bytes total.
Allocated during whole lifetime: 3295 blocks containing 16.8 MiB bytes total.
Sample cache size: 0 B
User name: user
Host Name: linux-upbm
Server Name: pulseaudio
Server Version: 0.9.12
Default Sample Specification: s16le 2ch 44100Hz
Default Sink: alsa_output.pci_8086_2668_sound_card_0_alsa_playback_0
Default Source: alsa_input.pci_8086_2668_sound_card_0_alsa_capture_0
Cookie: c7e5ab24
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kwrite.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_trash.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_file.so
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kwrite.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kwrite.so
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kwrite.so
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kio_http_cache_cleaner.so
X Error: BadAccess (attempt to access private resource denied) 10
Major opcode: 2 (X_ChangeWindowAttributes)
Resource id: 0x4c0001e
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kio_http_cache_cleaner.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
The program 'firefox' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
(Details: serial 1152 error_code 3 request_code 20 minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
The program 'firefox' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
(Details: serial 1157 error_code 3 request_code 20 minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kwrite.so
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
kdeinit4: preparing to launch /usr/lib/libkdeinit4_kio_http_cache_cleaner.so
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 20 (X_GetProperty)
Resource id: 0x4c0188e
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 20
Minor opcode: 0
Resource id: 0x4c0188e
kdeinit4: preparing to launch /usr/lib/kde4/kio_http.so
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::setCompositionMode: Painter not active
QPainter::end: Painter not active, aborted
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 20 (X_GetProperty)
Resource id: 0x4603cc8
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483633 destroyed
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 20
Minor opcode: 0
Resource id: 0x4603cc8
QThreadStorage: Thread 0x805b3d0 exited after QThreadStorage 2147483638 destroyed
nepomukserver: Fatal IO error: client killed
kdeinit4: Fatal IO error: client killed
kdeinit4: sending SIGHUP to children.
klauncher: Exiting on signal 1
kdeinit4: sending SIGTERM to children.
kdeinit4: Exit.
Unexpected response from KInit (response = 0).
startkde: Could not start ksmserver. Check your installation.
QProcess: Destroyed while process is still running.
kded4: Fatal IO error: client killed
kglobalaccel: Fatal IO error: client killed
Error: Can't open display: :0
kdeinit: Fatal IO error: client killed
klauncher: Exiting on signal 15
kded: Fatal IO error: client killed
kdeinit4: Fatal IO error: client killed
kdeinit4: sending SIGHUP to children.
ksmserver: Fatal IO error: client killed
kdeinit4: sending SIGTERM to children.
kdeinit4: Exit.
There are quite a lot of error messages, but there seem to be two major ones: one involving firefox and one with nepomukserver. What is this nepomukserver doing anyway?
The program 'firefox' received an X Window System error.
nepomukserver: Fatal IO error: client killed