Constant Contact forms submit creates Catchable fatal error

I'm having a problem trying to submit Constant Contact forms on my sites. They load properly on the page but when hitting submit, I get the following error:
Catchable fatal error: Object of class WP_Post could not be converted to string in /public_html/wp-includes/formatting.php on line 1031
I have looked at this line and it is function wp_check_invalid_utf8( $string, $strip = false ) {
$string = (string) $string;
That's where my knowledge ends - I have no idea what the issue is?