Empty error's box in wp-login.php

When an user that I forgot to activate tried to login, the error’s red box did not show any message (see image below).

In the debug.log I see:

[03-Jan-2013 10:50:55] PHP Notice: add_option was called with an argument that is deprecated since version 2.3 with no alternative available. in /web/htdocs/www.lascuoladiancel.it/home/cms/wp-includes/functions.php on line 2910
[03-Jan-2013 10:50:55] PHP Notice: load_plugin_textdomain was called with an argument that is deprecated since version 2.7 with no alternative available. in /web/htdocs/www.lascuoladiancel.it/home/cms/wp-includes/functions.php on line 2910
[03-Jan-2013 10:50:55] PHP Notice: Undefined index: action in /web/htdocs/www.lascuoladiancel.it/home/cms/wp-content/plugins/wordpress-password-register/wp-user-registration.php on line 90
[03-Jan-2013 10:50:55] PHP Notice: register_sidebar_widget è deprecata dalla versione 2.8! Utilizzare al suo posto wp_register_sidebar_widget(). in /web/htdocs/www.lascuoladiancel.it/home/cms/wp-includes/functions.php on line 2824
[03-Jan-2013 10:50:55] PHP Notice: ob_end_flush() [ref.outcontrol]: failed to delete buffer zlib output compression. in /web/htdocs/www.lascuoladiancel.it/home/cms/wp-includes/functions.php on line 2690

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Anyway the “Error message” in the settings page is compiled.

Any idea?

See:
https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-prn1/75316_10200341562126214_145693689_n.jpg