Cannot ‘un-exclude’ a category

Trying out the plugin. On the search tab I excluded a single category and the catgory ID number was shown and the search results excluded posts using the category. I then deleted the category from the search tag ‘excluded categories’ field (now blank) and saved the settings. However the catgory ID remained listed and the search still excluded the category.

Is there something else I need to do to un-exclude a category?