fmzac

This is not WP issue. this is due to contact form 7 changes.

In file cf7-campaignmonitor.php on line approx:161 replace whole function function wpcf7_cm_subscribe($obj) with this code

function wpcf7_cm_subscribe($obj)
{
	$cf7_cm = get_option( 'cf7_cm_'.$obj->id() );
	$submission = WPCF7_Submission::get_instance();

	if( $cf7_cm )
	{
		$subscribe = false;

		$regex = '/\[\s*([a-zA-Z_][0-9a-zA-Z:._-]*)\s*\]/';
		$callback = array( &$obj, 'cf7_cm_callback' );

		$email = cf7_cm_tag_replace( $regex, $cf7_cm['email'], $submission->get_posted_data() );
		$name = cf7_cm_tag_replace( $regex, $cf7_cm['name'], $submission->get_posted_data() );

		$lists = cf7_cm_tag_replace( $regex, $cf7_cm['list'], $submission->get_posted_data() );
		$listarr = explode(',',$lists);

		if( isset($cf7_cm['accept']) && strlen($cf7_cm['accept']) != 0 )
		{
			$accept = cf7_cm_tag_replace( $regex, $cf7_cm['accept'], $submission->get_posted_data() );
			if($accept != $cf7_cm['accept'])
			{
				if(strlen($accept) > 0)
					$subscribe = true;
			}
		}
		else
		{
			$subscribe = true;
		}

		for($i=1;$i<=20;$i++){

			if( isset($cf7_cm['CustomKey'.$i]) && isset($cf7_cm['CustomValue'.$i]) && strlen(trim($cf7_cm['CustomValue'.$i])) != 0 )
			{
				$CustomFields[] = array('Key'=>trim($cf7_cm['CustomKey'.$i]), 'Value'=>cf7_cm_tag_replace( $regex, trim($cf7_cm['CustomValue'.$i]), $submission->get_posted_data() ) );
			}

		}

		if( isset($cf7_cm['resubscribeoption']) && strlen($cf7_cm['resubscribeoption']) != 0 )
		{
			$ResubscribeOption = true;
		}
			else
		{
			$ResubscribeOption = false;
		}

		if($subscribe && $email != $cf7_cm['email'])
		{

			require_once(dirname(__FILE__).DIRECTORY_SEPARATOR.'csrest_subscribers.php');

			$wrap = new CF7CM_CS_REST_Subscribers( trim($listarr[0]), $cf7_cm['api'] );
			foreach($listarr as $listid)
			{
				$wrap->set_list_id(trim($listid));
				$wrap->add(array(
					'EmailAddress' => $email,
					'Name' => $name,
					'CustomFields' => $CustomFields,
					'Resubscribe' => $ResubscribeOption
				));
			}

		}

	}
}

basically these are the changes you need to take care if you are using any custom code related to contact form 7

/* Don't do this, since id property is no longer accessible. */
$id = $contact_form->id; // Wrong.

/* Use id() method instead. */
$id = $contact_form->id();

/* Properties form, mail, mail_2, messages, additional_settings are inaccessible as well. */
$form = $contact_form->form; // Wrong.

/* So, use prop() method to access them. */
$form = $contact_form->prop( 'form' );

/* To set the properties, use set_properties() method, like this: */
$mail = $contact_form->prop( 'mail' );
$mail['subject'] = "Well, hello, Dolly";
$contact_form->set_properties( array( 'mail' => $mail ) );

/* WPCF7_ContactForm object no longer has a posted_data property. */
$posted_data = $contact_form->posted_data; // Wrong.

/* Use WPCF7_Submission object's get_posted_data() method to get it. */
$submission = WPCF7_Submission::get_instance();

if ( $submission ) {
    $posted_data = $submission->get_posted_data();
}

i resolved this issue by modifying plugin.
on plugin installed location, edit file cf7-campaignmonitor.php

on approx line 40 replace this
add_action( ‘wpcf7_admin_before_subsubsub’, ‘add_cm_meta’ );

with
add_action( ‘wpcf7_add_meta_boxes’, ‘add_cm_meta’ );

Hope this will solve issue.