This bug is essentially the same as MDL-3992 but concerns the username not the password.

To summarise: LDAP authentication fails for users with quotes (') or other special charactors in their usernames.

Like bug MDL-3992 a solution is to add a stripslashes() call.

In auth_user_login() in auth/ldap/lib.php after the check for blank username || password add the following:
$username = stripslashes( $username );