function BUG: set_custom_wp_query_fields($fields, &$wp_query)

This function create query errors when $fields is one of ‘ids’, ‘id’, ‘all’ https://codex.www.ads-software.com/Class_Reference/WP_Query#Return_Fields_Parameter

So your code just return this as is instead to parse it like WordPress does on code query.php file, lines 2034 – 2043:

switch ( $q['fields'] ) {
			case 'ids':
				$fields = "$wpdb->posts.ID";
				break;
			case 'id=>parent':
				$fields = "$wpdb->posts.ID, $wpdb->posts.post_parent";
				break;
			default:
				$fields = "$wpdb->posts.*";
		}

Hopefully you can fix this ASAP as i already found multiple plugins / codes which get beaked due this.

https://www.ads-software.com/plugins/nextgen-gallery/