Solved Conflict Joomla 5.3 and Kunena 6.4.2

This is weird. If I view the Showcase when not logged in (ie Public) - the display is correct. If I log in then all the posts are placed in the left hand margin. See attachment. 

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.

1. Your forum is accessible to public. So first of all set your menu item forum to access registered.
2. Then create a new menu-item type alias for your public forum part.

The forum title must also remain secret? If this is the intention, you will have to accept that the index page for guests remains empty. If the categories are configured correctly, guests will only see the parent section and the permitted subcategories.
Example:
Main section = Public (name is your forum)
- Category = specific user group
- Category = specific user group
- Category = Public
Or you create a separate section for Publik, then it looks like this:
Extra Main section = Publik (name for guest forum)
- Category = Publik
In both cases you do not need to create extra menu items. Guests can only see the areas marked in green.

@rich as far as I understand public should acces the forum from public part of the website and not have to go to members area (which makes sense).

But following the path that @pwalker6 described earlier,  leads to a menu-item with alias showcat without Kunena menu,  so probably this menu-item isn't the correct type.

Well I'm not sure what you think this public link should be configured as, but the present one works and displays what I want. It's the display issue that is the problem and this only occurs when users are logged in.

Sorry,  can't help you with that as I can't login.

But your public menu item to the forum doesn't have a Kunena menu so your visitors can't navigate other than through the breadcrumbs,  and that eventually leads to an internal server error. But if you're happy with that......