Hi Joern.
If you want to add sidebar for only one page then you set the side bar from
Theme Options -> Design Options -> Default Layout Options = No sidebar
Then go to particular page/ post then click on screen options and tick on select layout for specific page / post then at the bottom of the page the same options appear
OK, if I set the default layout template as right sidebar, and then adjust all pages I don’t want to have a sidebar manually to show only no-sidebar, I can bypass the issue.