Issue with Ithinx Group plugin

Hello,

I’m having an issue with integration of the ithinx Group plugin.

It works fantastic… Until I add users to certain groups. Some work… Some do not. So I’m a little confused.

Below is from the error log. Is this an issue with the Mail Users plugin or with the Group plugin?

Thanks

PHP message: PHP   1. {main}() /www.644vgs.org/html/wp-admin/admin.php:0
PHP message: PHP   2. do_action() /www.644vgs.org/html/wp-admin/admin.php:236
PHP message: PHP   3. call_user_func_array:{/www.644vgs.org/html/wp-includes/plugin.php:525}() /www.644vgs.org/html/wp-includes/plugin.php:525
PHP message: PHP   4. mailusers_send_to_group_page() /www.644vgs.org/html/wp-includes/plugin.php:525
PHP message: PHP   5. require_once() /www.644vgs.org/html/wp-content/plugins/email-users/email-users.php:527
PHP message: PHP   6. include() /www.644vgs.org/html/wp-content/plugins/email-users/email_users_send_group_mail.php:247
PHP message: PHP   7. mailusers_get_itthinx_groups() /www.644vgs.org/html/wp-content/plugins/email-users/email_users_group_mail_form.php:169
PHP message: PHP   8. mailusers_get_recipients_from_itthinx_groups_group() /www.644vgs.org/html/wp-content/plugins/email-users/integration/itthinx-groups.php:52
PHP message: PHP Warning:  Invalid argument supplied for foreach() in /www.644vgs.org/html/wp-content/plugins/email-users/integra
2016/08/02 17:44:26 [error] 10346#10346: *652479 upstream sent too big header while reading response header from upstream, client: 10.189.254.8, server: 644vgs.org, request: "GET /wp-admin/admin.php?page=mailusers-send-to-group-page HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "644vgs.org", referrer: "http://644vgs.org/wp-admin/users.php"

https://wordpress.org/plugins/email-users/