404 error when using Index link to categories

I use the latest Joomla 4 with Kunena 6.1.3.2 and the NTS Kay template in the latest version (download today). 

Unfortunately I have a problem with the Kunena menu when I am already in the forum. 
If I click from my website into the forum, I see the category view (that's how it should be). 
Link to it is: website.de/forum.html

If I am then in the forum and click on the button in the menu "Index", so I see as hover effect below in the status bar also website.de/forum.html
 
But if I click on Index, I get a 404 error message and the browser says website.de/forum2/index.html .

Has someone any hints for me?
Thank you
Tom

Where can I set this? I do not find anything. In the Admin under Kunena menu I find nothing. 

had a wrong alias in menu