Discussed this in another thread, just isolated the cause. First found it in a clean RC5 install on a Crip theme. Same thing in default theme. It also happens on this board.
Problem: When I try to login with IE8, if session length is set to '60' on login page (or '1 hour' in header login), login fails ('Unable to login.... Check cookie settings'). Any other number/setting works. Login is normal in FF3.5.
It's in Login.template.php -
echo '
<dl>
<!--dt>', $txt['mins_logged_in'], ':</dt>
<dd><input type="text" name="cookielength" size="4" maxlength="4" value="', $modSettings['cookieTime'], '"', $context['never_expire'] ? ' disabled="disabled"' : '', ' class="input_text" /></dd-->
<dd><input type="hidden" name="cookielength" size="4" maxlength="4" value="61"', $context['never_expire'] ? ' disabled="disabled"' : '', ' class="input_text" /></dd>
<dt>', $txt['always_logged_in'], ':</dt>
<dd><input type="checkbox" name="cookieneverexp"', $context['never_expire'] ? ' checked="checked"' : '', ' class="input_check" onclick="this.form.cookielength.disabled = this.checked;" /></dd>';
As you can see, I've eliminated the session length option, and made it a hidden input set to '61'. It's not necessary, and this way user is either logged in for 61 min. by default, or checks 'Always stay logged in'. If you play with the number, you'll see that anything works except '60'.
No, it's not the browser's fault, and yes, many people use IE.