changing the content of the html-widget doesn’t work anymore

Since I updated my website to WP 4.9.0 I’m not able to edit the content of the WordPress built in widget “HTML” anymore. I used it in combination with this plugin to show it only on several pages.
When I add a new HTML-widget to a page I see the editor and can write my HTML-code and save it. But when I try to open the widget again to change something I only see the buttons for deleting and saving but not the editor. Does someone face the same problem and has an idea how to fix this?