Advanced > Additional CSS class broken (WP 5.1, Gutenberg)

Hi, thanks for Widget Shortcode!
In the block editor (WP 5.1), when I try to add an additional CSS class (under ‘Advanced’), the widget shortcode block in the editor is greyed out and an error is displayed: Error loading block: Invalid parameter(s): attributes.
Even removing the CSS class (and even re-saving the page) doesn’t remove the grey box.
Editing just the single block as HTML simply shows an empty field.
If I switch the whole editor to HTML, the block is still there. In the frontend, it’s still there, as well.