Login error 2.0.14 From Online Manual
This page is only relevant if you installed the 2.0.14 or 2.0.15 patch or upgrade, installed 2.0.14 or 2.0.15 directly, or installed a theme created prior to the release of 2.0.14 and started having problems with users logging into your forum. The error message which is shown will be a session timeout.
SMF 2.0.14+ now requires the a session check in the <form> for the login function. If your custom theme has a login form, such as in index.template.php, boardindex.template.php or login.template.php, you will need to modify the theme to allow the login function to work correctly. In order to modify the theme, add the following code along with the other "hidden" input types, or anywhere before the closing </form> tag in the login form.
<input type="hidden" name="', $context['session_var'], '" value="', $context['session_id'], '" />
This code is used to help increase security for SMF.