Xlib: unexpected async reply

[Matthias Kaeppler <noreply finitestate org>]

Hi,

I am displaying progress information about an asynchronous file transfer 
(using gnome-vfsmm) in a dialog box. However, I am getting very weird 
errors from the X Window System:

The program 'duality.elf' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadGC (invalid GC parameter)'.
  (Details: serial 233809 error_code 13 request_code 75 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.)

I tried reprodcing the error with --sync, but that didn't work, the 
program will just lock up and no error message at all will show up.

Sometimes I'm also getting this error, which is less verbose:

Xlib: unexpected async reply (sequence 0x71e6)!

I googled for this last error and it turned out that it may occur when 
the GUI library is not threading aware or not thread-safe. Can I enable 
threading support in gtkmm somewhere? I called Glib::thread_init() but 
that didn't help.

Thanks in advance,
Matthias