I heard from the elementor forum that the blog page cannot be directly customized with the basic version of Elementor,the option to edit archive templates is available only in our Pro version. I am guessing that the same goes for Yoast. I don’t think this is a problem because I can still get SEO information about individual posts with yoast.
Thanks for this. I will try editing the page with Elementor first and then setting it as the posts page.
Ok, done that. The fact that both plugins stop working suggests to me that there is a problem with the system underlying the plugins, but I might be wrong. Thanks for your suggestion.