[Solved] Notice: Undefined index: display_name
Please delete all caches in Dashboard > Forums > Dashboard admin page and try it again. Use other titles for topics, don't always use the Test word.
Also please navigate to My Profile > Account page and make sure you have Display Name. Set display name and save it. Then navigate to Dashboard > Users > Your Profile and make sure you have a Display Name.
I followed your steps and then tried to create my subject and I have this error that appeared:
[Subquery returns more than 1 row]
SELECT `slug` FROM apedys_wpforo_forums WHERE `forumid` =(SELECT forumid FROM `apedys_wpforo_topics` WHERE `topicid` =(SELECT `topicid` FROM `apedys_wpforo_posts` WHERE postid = 38689))
Warning: Cannot modify header information - headers already sent by (output started at /home/apedys/www/wp-includes/wp-db.php:1399) in /home/apedys/www/wp-includes/pluggable.php on line 1219
Have you deleted all caches in Dashboard > Forums > Dashboard admin page?