Appointments+ Test Connection Failure

Hi there,

I'm getting the following error trying to test the connection in Appointments+, "Warning: Cannot modify header information - headers already sent by
(output started at /var/sites/i/ionasdressmaking.com/public_html/wp-admin/includes/template.php:1642)
in /var/sites/i/ionasdressmaking.com/public_html/wp-includes/pluggable.php on line 876"

I've treid changing the theme to default, deactivating plugins. Can you help ?

Thanks