Can you please check your sites, if after last Joomla update (to 3.8.4.) all your menu items with search forms actually work? In my case they all stopped working and after pressing 'Submit' button instead of list of found items I see a blank component. It looks like if SEBLOD suddenly forgot where to display the search results.
Interesting thing - when I use SEBLOD search module that redirects to a menu position with SEBLOD search form/list, it works normally (but in the config options I pointed a menu item where it should display search results).