xfce4-screensaver does not supports PAM
Since commit ee7f451b PAM password authentication broke in xfce4-screensaver making it not possible to unlock screen.
The reason is that specifying --with-passwd-helper=$_libexecdir/xfce4-screensaver-chkpwd
seems to disable PAM support as only one authentication backed is supported and passwd-helper
is a different backend.