first: Shapely is a fantastic theme, I love it! And thanks for the great support!
My problem: I can’t add a sidebar to the page I have defined to display the posts. With all other pages it works fine when I add the sidebar using the page settings.
Thank you, that did it. I wasn’t aware that I had to set the default settings to “sidebar” and the to choose the “no sidebar” option for the pages that are not supposed to have one.