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 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.

SMF 2.0.14+ now requires the a session check in the <form> for the login function. The fix is simple, however. Just add in 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.