Can't play some DVD's in neither gxine nor Dragon Player

When opening gxine I get the following output:

warning: configuration item media.audio_cd.cddb_cachedir points to a non-existent location /home/kobilya/.xine/cddbcache
warning: configuration item media.capture.save_dir points to a non-existent location
warning: configuration item media.dvb.channels_conf points to a non-existent location /home/kobilya/.xine/channels.conf
warning: configuration item media.dvd.raw_device points to a non-existent location /dev/rdvd
warning: configuration item media.video4linux.radio_device points to a non-existent location /dev/radio0
warning: configuration item media.video4linux.video_device points to a non-existent location /dev/video0
warning: configuration item media.wintv_pvr.device points to a non-existent location /dev/video0
warning: configuration item decoder.external.real_codecs_path points to a non-existent location
warning: configuration item decoder.external.win32_codecs_path points to a non-existent location /usr/lib/codecs
warning: configuration item subtitles.separate.font_freetype points to a non-existent location
lirc: cannot initialise - disabling remote control
lirc: maybe lircd isn't running or you can't connect to the socket?

And then, when I try to open the DVD, it says the following, gets the entire window manager stuck for a few seconds, and then closes:

libdvdnav: Using dvdnav version 1.1.15 from http://xine.sf.net
libdvdread: Using libdvdcss version 1.2.10 for DVD access
libdvdread: Could not open /dev/dvd with libdvdcss.
libdvdnav: vm: faild to open/read the DVD
libdvdnav: Using dvdnav version 1.1.15 from http://xine.sf.net
libdvdread: Can't open /dev/dvd for reading
libdvdread: Using libdvdcss version 1.2.10 for DVD access
libdvdread: Could not open /dev/dvd with libdvdcss.
libdvdread: Can't open /dev/dvd for reading
libdvdnav: vm: faild to open/read the DVD
xine-lib: error: Read error from:: /dev/dvd
gxine: killed by watchdog bite
Aborted

In Dragon Player the situation is different: after I select "Play disc", it just ignores the selection, and the "Play" button stays disabled. I'm not sure if this is relevant, but here is the terminal output when dragon is launched:

dragonplayer(6002) Phonon::KdePlatformPlugin::createBackend: using backend: "GStreamer"
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableSubtitlesChanged()
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableAudioChannelsChanged()
Object::connect: No such signal Phonon::Gstreamer::MediaObject::titleChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableTitlesChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::chapterChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableChaptersChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::angleChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableAnglesChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableSubtitlesChanged()
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableAudioChannelsChanged()
Object::connect: No such signal Phonon::Gstreamer::MediaObject::titleChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableTitlesChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::chapterChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableChaptersChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::angleChanged(int)
Object::connect: No such signal Phonon::Gstreamer::MediaObject::availableAnglesChanged(int)

Thanks for your help,
Ilya