wellbeingtips on "[Plugin: WordPress SEO by Yoast] titles"

Idk how SEO titles can add something in your post titles. It shuld change just your SEO titles for searching engines.
Did you do some steps before? Do you use some other plugin for changing post titles or so? Did you do some changes to your single-post.php lines? Check your single-post.php file and there try to find this -