When trying to access site example.com, it opens normally. But there is a 404 in the site header. Log files contain no errors related to this behavior.
In case Wordpress debug log level is enabled, the site shows the following error:
404 Notice: The called constructor method for WP_Widget in ItemLikes_Widget is deprecated since version 4.3.0! Use __construct() instead. in /var/www/vhosts/example.com/httpdocs/wp-includes/functions.php on line 3846
This error is caused by the site code.
Contact example.com site developer in order to resolve the issue.