Can't start a specific remote X app any more

[luke dot kendall at cisra dot canon dot com dot au]

I recently upgraded to Cygwin 1.5.7(0.109/3/2), including
XWin release 4.3.0.56 installed from XFree86-bin-4.3.0-16.tar.bz2

Now when I try to run one particular remote X application (my MUA) I get
this error:

X Error of failed request:  BadAtom (invalid Atom parameter)
  Major opcode of failed request:  18 (X_ChangeProperty)
  Atom id in failed request:  0xee
  Serial number of failed request:  12
  Current serial number in output stream:  15

The remote (linux) machine has not had its sshd config changed since
2002.  It has "X11Forwarding yes", and used to work fine.

If I ssh -X into the machine, DISPLAY is set to localhost:10.0

The MUA runs fine on the Linux machine which I am connecting to with
ssh.  xmessage and other X applications I've tried also work okay.
The MUA is Postilion (a Tk/Tcl application built from the TkRat MUA,
but with the Nextstep Mail program's look and feel).

Other Tk applications work okay.

There's nothing odd in /tmp/XWin.log.

Any suggestions?

luke