Same here it shows everywhere where [newsletter] shortcode is used. I have also custom newsletter box on some parts of my website and when subscribing it shows white page with those errors:
Warning: Invalid argument supplied for foreach() in /home/r16023/domains/xxx/public_html/wp-content/plugins/newsletter/includes/module-base.php on line 401
Warning: Cannot modify header information – headers already sent by (output started at /home/r16023/domains/xxx/public_html/wp-content/plugins/newsletter/includes/module-base.php:401) in /home/r16023/domains/xxx/public_html/wp-content/plugins/newsletter/subscription/subscription.php on line 413
Warning: Cannot modify header information – headers already sent by (output started at /home/r16023/domains/xxx/public_html/wp-content/plugins/newsletter/includes/module-base.php:401) in /home/r16023/domains/xxx/public_html/wp-includes/pluggable.php on line 1427
Warning: Cannot modify header information – headers already sent by (output started at /home/r16023/domains/xxx/public_html/wp-content/plugins/newsletter/includes/module-base.php:401) in /home/r16023/domains/xxx/public_html/wp-includes/pluggable.php on line 1430
(site domain changed to xxx) However the email will be still subscribed, but user gets that error message.