Suddenly backend user search has stopped working.

For some reason, literally overnight, my user search in the backend has stopped working.

I go to this page: /wp-admin/users.php and do a search. The page reloads exactly as before with all users showing as before and the only thing which has changed is that the page is now /wp-admin/users.php?um_role

I’ve tried various plugins such as Improved User Search in Backend which also used to work fine before last night but which now doesn’t work fine.

Any ideas please since it’s now impossible to find users by their email or name, etc.