Autoloading failed to define function latex-mode

When I open some .tex files with emacs, I get the following message:

File mode specification error: (error "Autoloading failed to define function latex-mode")

and emacs is not is latex mode. The latex part of my init.el file is the following:

(require 'tex)
(setq TeX-PDF-mode 1)
(custom-set-variables
 '(TeX-view-program-list (quote (("Evince" "evince --page-index=%(outpage) %o")))))
(custom-set-faces)

I checked the installation of the auctex package already.

What is wrong?

Auctex does not provide tex, it provides auctex. Delete the (require 'tex) line, as it's already loaded inside auctex.

My auctex configuration for comparison:

;; AUCTeX
(setq TeX-auto-save t)
(setq TeX-parse-self t)
(setq-default TeX-master nil)

(add-hook 'LaTeX-mode-hook 'visual-line-mode)
(add-hook 'LaTeX-mode-hook 'flyspell-mode)
(add-hook 'LaTeX-mode-hook 'LaTeX-math-mode)

(add-hook 'LaTeX-mode-hook 'turn-on-reftex)
(setq reftex-plug-into-AUCTeX t)

(setq TeX-PDF-mode t)

;; Automatically activate folding mode in auctex, use C-c C-o C-b to fold.
(add-hook 'TeX-mode-hook
      (lambda () (TeX-fold-mode 1))); Automatically activate TeX-fold-mode.

;; Load german inspell if babel is german
;(add-hook 'TeX-language-de-hook
;      (lambda () (ispell-change-dictionary "german")))

(provide 'my_auctex)

which I load from my master config file with (require 'my_auctex).