Whenever I was updating old members, assigning them a role and expiration date I received this error:
Uncaught exception 'Ctct\Exceptions\CtctException' with message '[{"error_key":"json.max.items.violation","error_message":"#/email_addresses: 2 items were contained, but the maximum number of items allowed is 1."}]' in /httpdocs/wp-content/plugins/pmpro-constant-contact/includes/Ctct/Util/RestClient.php:93
#0 /httpdocs/wp-content/plugins/pmpro-constant-contact/includes/Ctct/Util/RestClient.php(48): Ctct\Util\RestClient::httpRequest('https://api.con...', 'PUT', Array, '{"id":"10634417...')
#1 /httpdocs/wp-content/plugins/pmpro-constant-contact/includes/Ctct/Services/ContactService.php(122): Ctct\Util\RestClient->put('https://api.con...', Array, '{"id":"10634417...')
#2 /httpdocs/wp-content/plugins/pmpro-constant-contact/includes/Ctct/ConstantContact.php(218): Ctct\Services\ContactService->updateContact('06847912-d102-4...', Object(Ctct\Components\Contacts\Contact), Array)
#3 /var
/httpdocs/wp-content/plugins/pmpro-constant-contact/includes/Ctct/Util/RestClient.php
This could be unrelated but the reason I was doing the above was because some members were listed as "Old Members" or Expired even though the expiration date was listed was "January 2016". I changed the role, expiration date and check Send Email To This User.