Question about captcha in DX Auth library

[eluser]Marco___[/eluser]
Hi all,
I'm in trouble with captcha, I have installed dx_auth with captcha enabled and recaptcha disabled.
Session in db is disabled too (just for more informations).
the problem is everytime I use captcha (registration,login,ecc) the $this->dx_auth->get_auth_error() method return this error: "Your confirmation code has expired. Please try again."
The captcha is visible and the file .png was created in the right folder, please can you help me?
Thanks in advance