Sandcastle: VS IDE index.html doesn't show content

Question

I'm using Sandcastle Visual Studio integration (VS 2022). I have a documentation project which creates a chm file. Everything works fine, all namespaces are listed. I don't use any special setup, the sfhbproj is default setup.

Now I want to switch to HTML documentation. I just switched to HTML output instead of *.CHM helpfile. I don't understand why the index.html isn't showing table of content of namespaces. In *.chm file everthing is listed by default, when generating html the sub htmls are created (\html folder) but not listet in index.html.

I also realised that search functionality doesn't work...

I searched YT tutorials and other threads but didn't find anything helpful. What must be done to get a working HTML documentation starting with index.html?

Basically I'd like to have same functionality in HTML like I had with CHM file.