It looks like gnome-session crashes before showing the dialog:

#0  0x00007f0f8e871d50 in strlen () from /lib/libc.so.6
No symbol table info available.
#1  0x00007f0f8e83a7ce in vfprintf () from /lib/libc.so.6
No symbol table info available.
#2  0x00007f0f8e8eff90 in __vasprintf_chk () from /lib/libc.so.6
No symbol table info available.
#3  0x00007f0f8f53677b in vasprintf (string=0x3c, 
    format=<value optimised out>, args=<value optimised out>)
    at /usr/include/bits/stdio2.h:199
No locals.
#4  IA__g_vasprintf (string=0x3c, format=<value optimised out>, 
    args=<value optimised out>)
    at /build/buildd/glib2.0-2.21.3/glib/gprintf.c:315
        len = <value optimised out>
        __PRETTY_FUNCTION__ = "IA__g_vasprintf"
#5  0x00007f0f8f522ff0 in IA__g_strdup_vprintf (
    format=0x3c <Address 0x3c out of bounds>, args=0x7fffe88a62c8)
    at /build/buildd/glib2.0-2.21.3/glib/gstrfuncs.c:244
        string = 0x0
#6  0x00007f0f8f52308d in IA__g_strdup_printf (
    format=0x3c <Address 0x3c out of bounds>)
    at /build/buildd/glib2.0-2.21.3/glib/gstrfuncs.c:270
        args = {{gp_offset = 16, fp_offset = 48, 
            overflow_arg_area = 0x7fffe88a6380, 
            reg_save_area = 0x7fffe88a62c0}}
#7  0x00000000004149e5 in gsm_logout_set_info_text (logout_dialog=0xfa9230, 
    seconds=60) at gsm-logout-dialog.c:273
        name = <value optimised out>
        info_text = 0x429c48 "You are currently logged in as \"%s\".\n"
        markup = <value optimised out>
        session_type = 0xef4ad0 ""
        __PRETTY_FUNCTION__ = "gsm_logout_set_info_text"
#8  0x0000000000414c7b in gsm_logout_dialog_show (logout_dialog=0x3c, 
    user_data=<value optimised out>) at gsm-logout-dialog.c:212
No locals.
#9  0x00007f0f8f9d662e in IA__g_closure_invoke (closure=0xfb8860, 
    return_value=0x0, n_param_values=1, param_values=0xfcf760, 
    invocation_hint=0x7fffe88a6510)
    at /build/buildd/glib2.0-2.21.3/gobject/gclosure.c:767
        marshal = 0x3c
        marshal_data = 0x646567676f6c2079
        __PRETTY_FUNCTION__ = "IA__g_closure_invoke"
#10 0x00007f0f8f9ec846 in signal_emit_unlocked_R (node=0xf02360, 
    detail=<value optimised out>, instance=<value optimised out>, 
    emission_return=<value optimised out>, 
    instance_and_params=<value optimised out>)
    at /build/buildd/glib2.0-2.21.3/gobject/gsignal.c:3247
        tmp = 0x7f0f8eb6d5b0
        handler = 0xfcb530
        accumulator = 0x0
        emission = {next = 0x0, instance = 0xfa9230, ihint = {signal_id = 12, 
            detail = 0, run_type = G_SIGNAL_RUN_FIRST}, state = EMISSION_RUN, 
          chain_type = 4}
        class_closure = 0xf022d0
        handler_list = 0xfcb530
        return_accu = <value optimised out>
        accu = {g_type = 0, data = {{v_int = 0, v_uint = 0, v_long = 0, 
              v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, 
              v_double = 0, v_pointer = 0x0}, {v_int = 0, v_uint = 0, 
              v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, 
              v_double = 0, v_pointer = 0x0}}}
        signal_id = 12
        max_sequential_handler_number = <value optimised out>
        return_value_altered = 1
#11 0x00007f0f8f9edc8e in IA__g_signal_emit_valist (instance=0xfa9230, 
    signal_id=<value optimised out>, detail=0, var_args=0x7fffe88a6700)
    at /build/buildd/glib2.0-2.21.3/gobject/gsignal.c:2980
        signal_return_type = 4
        param_values = <value optimised out>
        node = 0xf02360
        i = <value optimised out>
        n_params = <value optimised out>
        __PRETTY_FUNCTION__ = "IA__g_signal_emit_valist"
#12 0x00007f0f8f9ee213 in IA__g_signal_emit (instance=0x3c, signal_id=4365416, 
    detail=3901383368) at /build/buildd/glib2.0-2.21.3/gobject/gsignal.c:3037
        var_args = {{gp_offset = 24, fp_offset = 48, 
            overflow_arg_area = 0x7fffe88a67e0, 
            reg_save_area = 0x7fffe88a6720}}
#13 0x00007f0f9061c63b in IA__gtk_widget_show (widget=0xfa9230)
    at /build/buildd/gtk+2.0-2.17.4/gtk/gtkwidget.c:3009
        __PRETTY_FUNCTION__ = "IA__gtk_widget_show"
#14 0x0000000000420a5c in show_shutdown_dialog (manager=0xf3f020, 
    error=<value optimised out>) at gsm-manager.c:2889
        dialog = 0xfa9230
#15 gsm_manager_shutdown (manager=0xf3f020, error=<value optimised out>)
    at gsm-manager.c:3015
        __PRETTY_FUNCTION__ = "gsm_manager_shutdown"
#16 0x0000000000422b20 in dbus_glib_marshal_gsm_manager_BOOLEAN__POINTER (
    closure=0x7fffe88a6940, return_value=0x7fffe88a69e0, 
    n_param_values=<value optimised out>, param_values=0x13c3c60, 
    invocation_hint=<value optimised out>, marshal_data=0x420990)
    at gsm-manager-glue.h:234
        data1 = 0xf3f020
        data2 = 0x7fffe88a62c8
        v_return = <value optimised out>
        __PRETTY_FUNCTION__ = "dbus_glib_marshal_gsm_manager_BOOLEAN__POINTER"
#17 0x00007f0f90e4c592 in invoke_object_method (object=<value optimised out>, 
    object_info=<value optimised out>, method=0x42eeb0, 
    connection=<value optimised out>, message=<value optimised out>)
    at dbus-gobject.c:1347
        had_error = <value optimised out>
        send_reply = 1
        gerror = 0x0
        value_array = 0xfd7180
        return_value = {g_type = 20, data = {{v_int = 0, v_uint = 0, 
              v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, 
              v_double = 0, v_pointer = 0x0}, {v_int = 0, v_uint = 0, 
              v_long = 0, v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, 
              v_double = 0, v_pointer = 0x0}}}
        closure = {ref_count = 0, meta_marshal = 0, n_guards = 0, 
          n_fnotifiers = 0, n_inotifiers = 0, in_inotify = 0, floating = 0, 
          derivative_flag = 0, in_marshal = 0, is_invalid = 0, marshal = 0, 
          data = 0x0, notifiers = 0x0}
        in_signature = 0xfb7750 ""
        out_param_values = 0xfd1b80
        out_param_gvalues = 0xfc82c0
        out_param_count = 0
        out_param_pos = <value optimised out>
        out_param_gvalue_pos = <value optimised out>
        result = <value optimised out>
        reply = <value optimised out>
        have_retval = 1
        retval_signals_error = 1
        retval_is_synthetic = 1
        retval_is_constant = 0
        arg_metadata = 0xfd7180 "\2"
#18 0x00007f0f90e4cca5 in gobject_message_function (
    connection=<value optimised out>, message=<value optimised out>, 
    user_data=<value optimised out>) at dbus-gobject.c:1497
        pspec = <value optimised out>
        object = 0xf3f020
        setter = <value optimised out>
        getter = <value optimised out>
        getall = <value optimised out>
        s = <value optimised out>
        wincaps_propname = <value optimised out>
        wincaps_propiface = <value optimised out>
        iter = {dummy1 = 0x100000000, dummy2 = 0x7fffe88a6e1c, dummy3 = 15, 
          dummy4 = 0, dummy5 = 15951072, dummy6 = 0, dummy7 = 15958704, 
          dummy8 = 0, dummy9 = 0, dummy10 = 0, dummy11 = 15749408, pad1 = 0, 
          pad2 = 0, pad3 = 0x1}
        method = 0x42eeb0
        object_info = 0x42e620
        ret = <value optimised out>
#19 0x00007f0f90c1e19e in _dbus_object_tree_dispatch_and_unlock (
    tree=0xf2b9b0, message=0x100a720) at dbus-object-tree.c:856
        user_data = <value optimised out>
        next = 0x0
        path = 0xf05110
        exact_match = 0
        list = 0xf09e08
        link = <value optimised out>
        result = <value optimised out>
        subtree = <value optimised out>
#20 0x00007f0f90c11f8c in dbus_connection_dispatch (connection=0xf2bca0)
    at dbus-connection.c:4487
        message = 0x100a720
        link = 0x0
        filter_list_copy = 0x0
        message_link = 0xf09a60
        result = 60
        status = <value optimised out>
        __FUNCTION__ = "dbus_connection_dispatch"
#21 0x00007f0f90e49b65 in message_queue_dispatch (
    source=<value optimised out>, callback=<value optimised out>, 
    user_data=<value optimised out>) at dbus-gmain.c:101
        connection = 0xf2bca0
#22 0x00007f0f8f5019fe in g_main_dispatch (context=0xef7100)
    at /build/buildd/glib2.0-2.21.3/glib/gmain.c:1960
        dispatch = 0x7f0f90e49b50 <message_queue_dispatch>
        user_data = 0x0
        callback = 0
        cb_funcs = 0x0
        cb_data = 0x0
        current_source_link = {data = 0xf2da50, next = 0x0}
        source = 0xf2da50
        current = 0xf32660
        i = 0
#23 IA__g_main_context_dispatch (context=0xef7100)
    at /build/buildd/glib2.0-2.21.3/glib/gmain.c:2513
No locals.
#24 0x00007f0f8f5053c8 in g_main_context_iterate (context=0xef7100, 
    block=<value optimised out>, dispatch=<value optimised out>, 
    self=<value optimised out>)
    at /build/buildd/glib2.0-2.21.3/glib/gmain.c:2591
        max_priority = 0
        timeout = 0
        some_ready = 1
        nfds = 18
        allocated_nfds = -1890509040
        fds = <value optimised out>
        __PRETTY_FUNCTION__ = "g_main_context_iterate"
#25 0x00007f0f8f505825 in IA__g_main_loop_run (loop=0xf8d480)
    at /build/buildd/glib2.0-2.21.3/glib/gmain.c:2799
        self = 0xeda3d0
        __PRETTY_FUNCTION__ = "IA__g_main_loop_run"
#26 0x00007f0f904e9377 in IA__gtk_main ()
    at /build/buildd/gtk+2.0-2.17.4/gtk/gtkmain.c:1205
        tmp_list = 0x0
        functions = 0x0
        init = 0xf0f950
        loop = <value optimised out>
#27 0x000000000041b2ea in main (argc=1, argv=0x7fffe88a72b8) at main.c:527
        sa = {__sigaction_handler = {sa_handler = 0x1, sa_sigaction = 0x1}, 
          sa_mask = {__val = {0 <repeats 16 times>}}, sa_flags = 0, 
          sa_restorer = 0}
        error = 0x0
        display_str = <value optimised out>
        manager = <value optimised out>
        client_store = 0xf0f950
        xsmp_server = 0xf27e00
        signal_handler = <value optimised out>
        override_autostart_dirs = 0x0
        default_session_key = 0x0
        entries = {{long_name = 0x42bb92 "autostart", short_name = 97 'a', 
            flags = 0, arg = G_OPTION_ARG_STRING_ARRAY, arg_data = 0x636300, 
            description = 0x42be98 "Override standard autostart directories", 
            arg_description = 0x0}, {
            long_name = 0x42bb9c "default-session-key", short_name = 0 '\0', 
            flags = 0, arg = G_OPTION_ARG_STRING, arg_data = 0x6362f8, 
            description = 0x42bec0 "GConf key used to lookup default session", 
            arg_description = 0x0}, {long_name = 0x42bbb0 "debug", 
            short_name = 0 '\0', flags = 0, arg = G_OPTION_ARG_NONE, 
            arg_data = 0x6362ec, 
            description = 0x42bbb6 "Enable debugging code", 
            arg_description = 0x0}, {long_name = 0x42bbcc "failsafe", 
            short_name = 102 'f', flags = 0, arg = G_OPTION_ARG_NONE, 
            arg_data = 0x6362f0, 
            description = 0x42bef0 "Do not load user-specified applications", 
            arg_description = 0x0}, {long_name = 0x42bbd5 "version", 
            short_name = 0 '\0', flags = 0, arg = G_OPTION_ARG_NONE, 
            arg_data = 0x6362e8, 
            description = 0x42bbdd "Version of this application", 
            arg_description = 0x0}, {long_name = 0x0, short_name = 0 '\0', 
            flags = 0, arg = G_OPTION_ARG_NONE, arg_data = 0x0, 
            description = 0x0, arg_description = 0x0}}