Thanks for your answer but I don't think that this error is on my configuration or PAM modules. Other programs (login, wdm, passwd, sudo, su, etc.) work correctly, xscreensaver no.
Did xscreensaver work before you started messing with your PAM config? Then your PAM config is what broke it.
When you figure out what you did wrong, I'm reasonably certain the answer will be that you didn't edit pam.d/xscreensaver properly.
I can't tell you what the right thing to put there is in your case, because I didn't write PAM.
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