The stock player discards some useful error messages. dbus-monitor --session | grep -A 7 error You should get something with mafw in it, an error, a number, and a string containing details about the error.
Nokia-N900:~# dbus-monitor --session |grep -A 7 error string "type='error'" method call sender=:1.147 -> dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer serial=116 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.renderer; member=get_status method return sender=:1.45 -> dest=:1.147 reply_serial=116 uint32 1 uint32 0 int32 0 string "urisource::http://www.youtube.com/watch?v=ZUJGl...e_gdata_player" method call sender=:1.147 -> dest=com.nokia.mafw.playlist serial=117 path=/com/nokia/mafw/playlist; interface=com.nokia.mafw.playlist; member=create_playlist -- string "error-policy" variant uint32 1 signal sender=:1.45 -> dest=(null destination) serial=234 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.extension; member=property_changed string "error-policy" variant uint32 1 method call sender=:1.147 -> dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer serial=124 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.renderer; member=stop method return sender=:1.45 -> dest=:1.147 reply_serial=124 method call sender=:1.147 -> dest=com.nokia.mafw.playlist serial=125 path=/com/nokia/mafw/playlist/1; interface=com.nokia.mafw.playlist; member=get_size method return sender=:1.76 -> dest=:1.147 reply_serial=125 uint32 1 method call sender=:1.147 -> dest=com.nokia.mafw.playlist serial=126 path=/com/nokia/mafw/playlist/1; interface=com.nokia.mafw.playlist; member=clear -- error sender=:1.147 -> dest=:1.144 error_name=org.freedesktop.DBus.Error.UnknownMethod reply_serial=17 string "Method "mime_open" with signature "s" on interface "com.nokia.mediaplayer" doesn't exist " method call sender=:1.147 -> dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer serial=132 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.renderer; member=get_status method return sender=:1.45 -> dest=:1.147 reply_serial=132 uint32 1 uint32 4294967295 int32 0 -- signal sender=:1.45 -> dest=(null destination) serial=246 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.extension; member=error string "com.nokia.mafw.error.renderer" int32 15 string "text/html decoder" method call sender=:1.147 -> dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer serial=144 path=/com/nokia/mafw/renderer/gstrenderer; interface=com.nokia.mafw.renderer; member=stop method return sender=:1.45 -> dest=:1.147 reply_serial=144 method call sender=:1.147 -> dest=org.maemo.Playback.Manager serial=145 path=/org/maemo/Playback/Manager; interface=org.maemo.Playback.Manager; member=RequestState object path "/org/maemo/playback1" string "Stop"