Solved Sample Data not installed

You have to assign the modules correctly. For example: Main menu is the public menu and should not be displayed in the forum. Then you have to open this module and deselect all menu items for the forum (see image).
Then open the module for the User menu and select all menu items for the forum.
You have to do this with all modules which are different between public and registered.
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It's exactly the same and that's why I'm surprised, even though the main menu is inactive in the forum section, the main menu is still displayed. Please see the attached image.

1. this image is not helpful because I can't see if "Only on the selected pages" or "On all pages except the selected" is selected. Correct would be the first one in this case.
2. empty all caches (Joomla and browser).
3. if your Joomla template uses page optimization (compression), disable it, otherwise the data will be loaded from this compressed file.
In general, optimization tools should be disabled during setup, as they can falsify the result.
4. also check all other modules in the puplic area if Kunena menu items are assigned.

You are right.
I use a separate template for each section.
The forum menu was not activated in the template settings, and therefore the forum was displayed in the public section.
The problem was solved and  Thank you so much for your help.