Ever since upgrading I have ran into the same problem on different servers with different themes. The issue is the login widget on each of these themes (by different authors, so it isn't the theme's issue)
Gives me an error of
(https://i.gyazo.com/c06908f4eebd9d022a98e1b692a323f1.png)
Your session timed out while posting. Please go back and try again.
I have attempted to do fixes, I am running Php 5.6, 7, and 7.1 depending on my different installations.
I have also tried to do a brand new install and that didn't fix it either.
(https://i.gyazo.com/3fedfc063de9e7957810a9c6a22c0480.png)
I have also has other users test this from my forum and they got the exact same results so it isn't anything from my end.
This is using 2.0.14
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
The 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.
Thank you!