Just started seeing this on a client site that was using the “extreme seo” plugin. Shutting that off resolved it here.
That plugin was spitting out gibberish the other day as well due to a malformed php file on the site that plugin calls home to so I’m recommending my client stay away from it.
macmanx’ suggestion of shutting off all plugins is a good place to start for sure.
Otherwise, try repairing wp_posts and wp_postmeta.