Normally when I've seen this it's when you are using a variable like a normal string when in fact it's actually an array such as this example:
[Tue Mar 13 04:22:35 2018] [error] PHP Catchable fatal error: Object of class WP_Term could not be converted to string in /vhost/httpdocs/wp-content/plugins/wp-instagram-post/classes/class-woo-igp.php on line 578
&nbs........