Hi @lengerke, thanks for reaching out and trying your best to explain the problem.
It sounds like YARPP is trying to find related pages on each page, but isn’t finding any.
It might be a bit premature, but I have a guess what the problem is.
In YARPP settings (from the WordPress admin dashboard, under the “Settings” admin menu, on the “YARPP” sub-menu item) in the “The Pool” options, is “Display Results from all post types” checked? (Screenshot) I’m guessing it’s not.
Make sure that setting is checked and then save. That will tell YARPP to show similar posts (and pages, and any other custom post type) on pages, isntead of only looking for similar pages.
Does that make sense?