Tomcho’s response worked for me – it generates the nonce code and log you out…
Have you checked here yet?
https://codex.www.ads-software.com/Template_Tags/wp_logout_url
My problem is that I’ve already done this change and I still can’t log out.
If you don’t want to go there the code is:
<a href="<?php echo wp_logout_url(); ?>">Logout</a>
