1. Search for wp_enqueue_script calls. Probably you have added jquery-ui to your code. Be careful with it. Check out the folder at /wp-includes/jquery/ui. Use already registered scripts like:
wp_enqueue_script( 'jquery-ui-sortable' );
wp_enqueue_script( 'jquery-ui-tabs' );
wp_enqueue_script( 'jquery-ui-slider' );
wp_enqueue_script( 'jquery-ui-datepicker' );
// TRY TO AVOID:
// wp_enqueue_script( 'jquery-ui-core', '/js/jquery-ui/jquery-ui.min.js' );
2. Deactivate plugins temporarily you have added or upgraded recently to see if this resolves the problem. If this works, re-activate them one-by-one to find the bad code.