It is Inovado theme and it is caused by this function:
function new_excerpt_length($length) {
global $data;
return $data['text_excerptlength'];
}
add_filter('excerpt_length', 'new_excerpt_length');It looks like Newsletter does not have access to $data['text_excerptlength']. When I change it to return 20; then everything works well. But it is strange because all other plugins and theme functions works well with these data, only Newsletter has problems (and not on preview page).
Thank you for your help!