Running:

gdb gnome-panel
set env GDK_SYNCHRONIZE 1
run --replace
bt full

gives me

[New Thread 0x7fffcad31700 (LWP 14075)]
[Thread 0x7fffd0b7a700 (LWP 14065) exited]

(gnome-panel:14057): Gdk-ERROR **: 18:34:13.987: The program 'gnome-panel' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadImplementation (server does not implement operation)'.
  (Details: serial 42519 error_code 17 request_code 20 (core protocol) 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 GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

Thread 7 "gnome-panel" received signal SIGTRAP, Trace/breakpoint trap.
[Switching to Thread 0x7fffcad31700 (LWP 14075)]
0x00007ffff4eb3ea1 in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0

#0  0x00007ffff4eb3ea1 in  () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007ffff4eb6819 in g_log_writer_default () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007ffff4eb4a8e in g_log_structured_array ()
    at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007ffff4eb54ce in g_log_structured_standard ()
    at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007ffff69bfc41 in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#5  0x00007ffff69ccac3 in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#6  0x00007ffff4b6a8fa in _XError () at /usr/lib/x86_64-linux-gnu/libX11.so.6
#7  0x00007ffff4b4e52c in XGetWindowProperty () at /usr/lib/x86_64-linux-gnu/libX11.so.6
#8  0x00007ffff69d0544 in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#9  0x00007ffff69cd1ad in  () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#10 0x00007ffff6fe35dd in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#11 0x00007ffff6fe8831 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#12 0x00007ffff6fecab5 in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#13 0x00007ffff518910d in g_closure_invoke () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#14 0x00007ffff519c12e in  () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#15 0x00007ffff51a4715 in g_signal_emit_valist ()
    at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#16 0x00007ffff51a512f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#17 0x00007ffff6fdcee6 in gtk_widget_realize () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#18 0x00007ffff6feadad in  () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#19 0x00007ffff518910d in g_closure_invoke () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#20 0x00007ffff519c12e in  () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#21 0x00007ffff51a4715 in g_signal_emit_valist ()
    at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#22 0x00007ffff51a512f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#23 0x00007ffff6fd70c6 in gtk_widget_show () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#24 0x00007fffca30e13b in Popup::show(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) () at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#25 0x00007fffca300f63 in AppletGUI::show_dialog_popup() ()
    at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#26 0x00007fffca30113a in AppletGUI::update(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) () at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#27 0x00007fffca312702 in AppletGnome::update(int) ()
    at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#28 0x00007fffca2d31a3 in Local::fam_monitoring() ()
    at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#29 0x00007fffca2d3ad6 in Local::fam_start_monitoring() ()
    at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#30 0x00007fffca2d3dc5 in Local::start() ()
    at /usr/lib/x86_64-linux-gnu/gnubiff/libgnubiff-applet.so
#31 0x00007ffff4ed6175 in  () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007ffff47076db in start_thread (arg=0x7fffcad31700) at pthread_create.c:463
        pd = 0x7fffcad31700
        now = <optimised out>
        unwind_buf = 
              {cancel_jmp_buf = {{jmp_buf = {140736596219648, 4513413494597600125, 140736596216960, 0, 93824995200480, 140737488344816, -4513372021984214147, -4513392660002258051}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = <optimised out>
#33 0x00007ffff443088f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(gdb) quit
A debugging session is active.

	Inferior 1 [process 14057] will be killed.

Quit anyway? (y or n) y