In action.php file (plugin discussion), you have to change :

if (!empty($comment['replies'])) by if (empty($comment['replies']))

Normally, action.php is present in /lib/plugins/discussion/action.php.

If you struggle to find it, you can find action.php in your dokuwiki folder with the following command line :

sudo find <your_dokuwiki_folder> -name action.php

You will have this kind of result :

dokuwiki/config/dokuwiki/lib/plugins/popularity/action.php
dokuwiki/config/dokuwiki/lib/plugins/action.php
dokuwiki/config/dokuwiki/lib/plugins/extension/action.php
dokuwiki/config/dokuwiki/lib/plugins/smtp/action.php
dokuwiki/config/dokuwiki/lib/plugins/captcha/action.php
dokuwiki/config/dokuwiki/lib/plugins/safefnrecode/action.php
dokuwiki/config/dokuwiki/lib/plugins/acl/action.php
dokuwiki/config/dokuwiki/lib/plugins/logviewer/action.php
dokuwiki/config/dokuwiki/lib/plugins/styling/action.php
dokuwiki/config/dokuwiki/lib/plugins/discussion/action.php
dokuwiki/config/dokuwiki/lib/plugins/authad/action.php

And :

sudo nano dokuwiki/config/dokuwiki/lib/plugins/discussion/action.php

Source(s) / Useful link(s) : https://github.com/dokufreaks/plugin-discussion/issues/349