Solved Conflict Joomla 5.3 and Kunena 6.4.2

Yes. That's what I saw as well. So that's why I suggested to first fix your menu's. So fix your menu's as suggested and set your home menu-item from your mainmenu as default again.

OK solved the home page problem - my bad. Now the Layouts category is showing the 24 topics (what changed?). So I seem to be left with the post display problem - which was where I started. Appreciate your help. The whole website is being server-upgraded tomorrow so may be offline for a while,

Almost there. Your index page is still not showing public categories, for which I don't have a solution right now, but will check on my own site which setting causes this.
And can you post a screenshot from your layout problem?

Your index page is still not showing public categories

Check your category display settings as already mentionned by @rich

Pinkeltje wrote:

Your index page is still not showing public categories

Check your category display settings as already mentionned by @rich
 

The same can also happen if the parent section is only allowed for a specific member group.
In such a case, this permission is configured incorrectly. It makes no sense to allow a parent section only for certain users if it also contains categories that should be accessible to everyone.

As I've said before, this is intentional - the public don't have access to the Forum, except for one sub-category, which I suggest you access to see the display problem. Go to the home page, then either go About Us > Modelling or scroll down on home page to Modelling. Then click the sub-menu item showcase or the link Summer 2021 modelling Showcase.