Create Kunena post on new user registration

Thank you. I can confirm that the error no longer shows, and that a new topic is created in the forum category of my choice.

However, it seems I still did something wrong. When an account is created, the forum post does not post the details (username, email, etc...) of the new user, but instead of my own account. 

I used this (as you can also see in the screenshot in my original post):

ID: [user_id]
Username: [username]
Display Name: [name]
Real Name: [cb_realname]
E-mail: 

In the Group Leader field I entered the ID of my own account as that's the account the posts should be made with in the forum. When I leave that empty, I get an error and no post is created.

What am I doing wrong?

Sounds like you set the "Owner" and by doing so it will cause the substitutions to be from that user as the parameter states. You can pull in variables directly from a trigger or refer to the action user using substitutions. Below are two examples.

Variables:
Username: [var1_username]
Username: [cb:userdata field="username" user="[var1_id]" /]

Action User:
Username: [cb:userdata field="username" user="[action_user]" /]

Thank you for this. I used the method per your second example (Action User) and now it works perfectly as intended!