Problema de logueo persistente en el foro

<!-- login form -->
        <form action="login.php?do=login" method="post" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, $show[nopasswordempty])">
        <script type="text/javascript" src="clientscript/vbulletin_md5.js?v=$vboptions[simpleversion]"></script>
        <table cellpadding="0" cellspacing="$stylevar[formspacer]" border="0">
        <tr>
            <td class="smallfont" style="white-space: nowrap;"><label for="navbar_username">$vbphrase[username]</label></td>
            <td><input type="text" class="bginput" style="font-size: 11px" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="$vbphrase[username]" onfocus="if (this.value == '$vbphrase[username]') this.value = '';" /></td>
            <td class="smallfont" nowrap="nowrap"><label for="cb_cookieuser_navbar"><input type="checkbox" name="cookieuser" value="1" tabindex="103" id="cb_cookieuser_navbar" accesskey="c" />$vbphrase[remember_me]</label></td>
        </tr>
        <tr>
            <td class="smallfont"><label for="navbar_password">$vbphrase[password]</label></td>
            <td><input type="password" class="bginput" style="font-size: 11px" name="vb_login_password" id="navbar_password" size="10" tabindex="102" /></td>
            <td><input type="submit" class="button" value="$vbphrase[log_in]" tabindex="104" title="$vbphrase[enter_username_to_login_or_register]" accesskey="s" /></td>
        </tr>
        </table>
        <input type="hidden" name="s" value="$session[sessionhash]" />
        <input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
        <input type="hidden" name="do" value="login" />
        <input type="hidden" name="vb_login_md5password" />
        <input type="hidden" name="vb_login_md5password_utf" />
        </form>
        <!-- / login form -->