You installed the 2.0.14 update or installed 2.0.14 directly and started having problems with users logging into your forum

If your custom theme has a login form such as in the index.template.php or boardindex.template.php or in the login.template.php

SMF 2.0.14+ now requires the following code added in the <form> tag just add in the closing </form> tag in any login form.

<input type="hidden" name="', $context['session_var'], '" value="', $context['session_id'], '" />

This code is used to help increase security for SMF.