After two days of research I have to ask now. Sigh.
I use the free version of pagelines/platform. I deactivated plugins before and switched to another theme as well. I also made sure that in "discussion" all necessary boxes are ticked, eg allow comments and so forth but still: there is no leave a comment/leave a reply link or form on the bottom of my post page or the single post. I have three pages: About, Shots, Contact and on About and Contact I can at least see the social media icons for Twitter and Facebook. On my Shots (post page/blog page) there is nothing! When I activate another theme I could leave a comment on the About and Contact page but not on Shots - the only page where it´s necessary! I am not sure but I assume that it has something the do with wordpress as well as with the pagelines theme? I am getting really desperate :((((
Thanks for all your help!