I managed to get a backtrace by sending HUP to a xemacs-21.5-b1 process
that was in an infloop. Here it is,

milhouse$ xemacs-21.5-b1

[1]+  Stopped                 xemacs-21.5-b1
milhouse$ bg
[1]+ xemacs-21.5-b1 &
milhouse$ kill -1 %1
milhouse$
Fatal error (1).

Your files have been auto-saved.
Use `M-x recover-session' to recover them.

If you have access to the PROBLEMS file that came with your
version of XEmacs, please check to see if your crash is described
there, as there may be a workaround available.
Otherwise, please report this bug by running the send-pr
script included with XEmacs, or selecting `Send Bug Report'
from the help menu.
As a last resort send ordinary email to `crashes@xemacs.org'.
*MAKE SURE* to include the information in the command
M-x describe-installation.

If at all possible, *please* try to obtain a C stack backtrace;
it will help us immensely in determining what went wrong.
To do this, locate the core file that was produced as a result
of this crash (it's usually called `core' and is located in the
directory in which you started the editor, or maybe in your home
directory), and type

  gdb /usr/local/bin/xemacs-21.5-b1.exe core

then type `where' when the debugger prompt comes up.
(If you don't have GDB on your system, you might have DBX,
or XDB, or SDB.  A similar procedure should work for all of
these.  Ask your system administrator if you need more help.)

Lisp backtrace follows:

  looking-at("--text follows this line--$\\|[   ]*$\\|-- $\\|---+$\\|^?$\\|.*wro
te:$\\|\\(\\([  ]*\\(\\w\\|[-_.]\\)+>+\\|[      ]*[]>»|:}+]\\)+\\)[     ]*$")
  # bind (quoted point beg end leading-space bolp not-break arg)
  message-newline-and-reformat(nil t)
  # bind (arg)
  message-fill-paragraph(nil)
  # bind (function fill-paragraph-function arg)
  #<compiled-function (arg) "...(54)" [function before end start arg fill-paragr
aph-function nil forward-paragraph newline 1 backward-paragraph fill-region fill
-region-as-paragraph use-hard-newlines] 5 1025528 (list (if current-prefix-arg .
..))>(nil)
  apply(#<compiled-function (arg) "...(54)" [function before end start arg fill-
paragraph-function nil forward-paragraph newline 1 backward-paragraph fill-regio
n fill-region-as-paragraph use-hard-newlines] 5 1025528 (list (if current-prefix
-arg ...))> nil)
  # bind (args function)
  filladapt-funcall(fill-paragraph nil)
  # bind (paragraph-ignore-fill-prefix adaptive-fill-mode adaptive-fill-regexp c
omment-multi-line fill-prefix retval)
  # (unwind-protect ...)
  byte-code("..." [function done retval sign delta fill-column nil t filladapt-a
dapt 1 0 filladapt-funcall filladapt-paragraph-within-fill-tolerance success run
-hooks filladapt-fill-paragraph-post-hook throw arg low high lim fill-prefix old
-fill-column filladapt-mode comment-multi-line adaptive-fill-regexp adaptive-fil
l-mode paragraph-ignore-fill-prefix filladapt-fill-column-tolerance filladapt-fi
ll-column-backward-fuzz filladapt-fill-column-forward-fuzz] 7)
  # (catch done ...)
  # bind (arg function)
  filladapt-fill-paragraph(fill-paragraph nil)
  # bind (filladapt-inside-filladapt arg)
  fill-paragraph(nil)
  # bind (command-debug-status)
  call-interactively(fill-paragraph)
  # bind (arg)
  fill-paragraph-or-region(nil)
  # bind (command-debug-status)
  call-interactively(fill-paragraph-or-region)
  # (condition-case ... . error)
  # (catch top-level ...)