PHP Notice: Array to String Conversion in formatting.php

I get this on frontend of my site 6 times in a row (http:sunnmorsk-klatreklubb.no):

Notice: Array to string conversion in /home/sunnmaug/public_html/wp-includes/formatting.php on line 703

When I look it up in the file it is from this (pasted line 702 to 706, 703 is the second line of text):

function wp_check_invalid_utf8( $string, $strip = false ) {
	$string = (string) $string;

	if ( 0 === strlen( $string ) ) {
		return '';

Anyone know how to fix this? The notice showed up after I updated the custom Facebook feed pro plugin, but the notice is still there after disabling the plugin.