dired-backup-diff can't find file

["Luis O. Silva" <silva@paloma.spbu.ru>]

Dear Emacs Community,

I use Emacs 21.1.1 on Linux RH7.2.  Recently I changed the way
Emacs processes backup files putting the following in my
.emacs file:

(setq backup-directory-alist 
      '((".+\\.tex$" . "~/backup/tex")
        ("/[^\\./]+\\..+$" . "~/backup/code")
        ("/[^\\.]+$" . "~/backup")
        ("/\\.." . "~/backup/dotfiles")))
;; backup variables
(setq 
 version-control t
 kept-new-versions 5
 kept-old-versions 5
)

All works fine and I was completely satisfied with this
setup. However I can't use comparison of backup files in
Dired. The function dired-backup-diff works fine when it
compares the current file with its latest backup file. In this
case, Emacs correctly finds the backup file in its
correspondent directory. But, when the current file is itself a
backup one, dired-backup-diff can't find the original file. It
tries to find it in the current directory under an incorrect
name and Diff exits abnormally.

Is this a bug? May it be that I must do some other changes?
Please help me to find out how to solve this problem, I'd be
infinitely grateful.

Regards,
luis

-- 
Luis Octavio Silva P.
St. Petersburg State University.
66/3 Botanicheskaya St., Apt.119/2
Stary Peterhof
St. Petersburg, Russia.