[AUCTeX] Error loading a plain TeX file

[Julien Cubizolles]

Whenever I open a plain TeX file, I get the following error. I'm not
sure where the error lies (ido, AucTeX). Any idea what is going wrong ?

Debugger entered--Lisp error: (error "Autoloading failed to define function 
plain-tex-mode")
  plain-tex-mode()
  tex-mode()
  set-auto-mode-0(tex-mode nil)
  set-auto-mode()
  normal-mode(t)
  after-find-file(nil t)
  find-file-noselect-1(#<buffer commandes-auteur.tex> 
"~/vuibert/vuibert-sup/commandes-auteur.tex" nil nil 
"~/vuibert/vuibert-sup/commandes-auteur.tex" (2605063 2056))
  find-file-noselect("~/vuibert/vuibert-sup/commandes-auteur.tex" nil nil)
  ido-file-internal(raise-frame)
  ido-find-file()
  call-interactively(ido-find-file nil nil)
  command-execute(ido-find-file)