A few members have reported to me that they cannot log in.
They say that it says
"The image verification code that you entered was incorrect. Please enter the code exactly how it appears in the image."
But there is no capatcha.
Upgrade to 1.4.6, this was fixed in 1.4.5.
Then make sure you followed the 1.4.4 upgrade steps, specifically the template edits, and also that you have the latest member.php. This bug was fixed about 6 months ago, very soon after 1.4.4 was released.
I did, I did a clean installion of 1.6
So your member_login template is exactly this??
<html>
<head>
<title>{$mybb->settings['bbname']} - {$lang->login}</title>
{$headerinclude}
</head>
<body>
{$header}
<br />
{$inline_errors}
{$member_loggedin_notice}
<form action="member.php" method="post">
<table border="0" cellspacing="{$theme['borderwidth']}" cellpadding="{$theme['tablespace']}" class="tborder">
<tr>
<td class="thead" colspan="2"><strong>{$lang->login}</strong></td>
</tr>
<tr>
<td class="trow1"><strong>{$lang->username}</strong></td>
<td class="trow1"><input type="text" class="textbox" name="username" size="25" maxlength="{$mybb->settings['maxnamelength']}" style="width: 200px;" value="{$username}" /></td>
</tr>
<tr>
<td class="trow2"><strong>{$lang->password}</strong><br /><span class="smalltext">{$lang->pw_note}</span></td>
<td class="trow2"><input type="password" class="textbox" name="password" size="25" style="width: 200px;" value="{$password}" /> (<a href="member.php?action=lostpw">{$lang->lostpw_note}</a>)</td>
</tr>
{$captcha}
</table>
<br />
<div align="center"><input type="submit" class="button" name="submit" value="{$lang->login}" /></div>
<input type="hidden" name="action" value="do_login" />
<input type="hidden" name="url" value="{$redirect_url}" />
</form>
{$footer}
</body>
</html>
If it is, you'll be the first person saying it isn't working on a new installation, the fix has been confirmed 100+ times.