An interesting discovery: system emails are sent with text/html; charset="UTF-8", but newsletters with text/plain; charset="iso-8859-1". How can I change it? I need to use utf-8 only.
Couple of things to check: 1) your language file has the correct encoding set (ie. 'encoding' => 'ISO-8859-1', in your amember\language\* file that you are using) 2) amember\includes\phpmailer\class.phpmailer.php has a char set defined as well as amember\common.inc.php