The problem are existing entries in the database where the content is no json, but the rich editor tries to handle them. Could be in Discussion, Comment, Draft table
Ready! the change it will be at themes/cloudy/design/steam/overdose.css, then search .Back inside the file, delete all content inside the {} and put display:none
Hi @TwistedTwigleg just one thing: you have forgotten to Gdn::translate'Search in discussion contents and title'and 'All Discussions' in /view/index.php