WP Members 플러그인이 업데이트 되면서 username 필드를 필수로 설정한 듯합니다.

FTP로 접속해서 /wp-content/plugins/cosmosfarm-members/class/Cosmosfarm_Members.class.php 파일에

아래의 코드를 찾아서

public function register_data($fields, $toggle){
	$option = get_cosmosfarm_members_option();
	if($toggle == 'new'){
		if($option->allow_email_login){
			if(is_multisite()){
				$fields['username'] = sanitize_user($fields['user_email']);
			}

			$user_nicename = isset($_POST['user_nicename']) ? sanitize_user($_POST['user_nicename']) : '';
			$display_name = isset($_POST['display_name']) ? sanitize_user($_POST['display_name']) : '';
			$nickname = isset($_POST['nickname']) ? sanitize_user($_POST['nickname']) : '';

			$user_email = explode('@', $fields['user_email']);
			$display = reset($user_email);
			$display = sanitize_user($display);

			$fields['user_nicename'] = $user_nicename ? $user_nicename : $display;
			$fields['display_name'] = $display_name ? $display_name : $display;
			$fields['nickname'] = $nickname ? $nickname : $display;
		}
	}
	else if($toggle == 'edit'){
		unset($fields['username']);
	}
	return $fields;
}

아래의 코드로 교체해보시겠어요?

public function register_data($fields, $toggle){
	$option = get_cosmosfarm_members_option();
	if($toggle == 'new'){
		if($option->allow_email_login){
			
			add_action('user_register', array($this, 'user_register_allow_email_login'));
			
			$user_nicename = isset($_POST['user_nicename']) ? sanitize_user($_POST['user_nicename']) : '';
			$display_name = isset($_POST['display_name']) ? sanitize_user($_POST['display_name']) : '';
			$nickname = isset($_POST['nickname']) ? sanitize_user($_POST['nickname']) : '';

			$user_email = explode('@', $fields['user_email']);
			$display = reset($user_email);
			$display = sanitize_user($display);

			$fields['user_nicename'] = $user_nicename ? $user_nicename : $display;
			$fields['display_name'] = $display_name ? $display_name : $display;
			$fields['nickname'] = $nickname ? $nickname : $display;
		}
	}
	else if($toggle == 'edit'){
		unset($fields['username']);
	}
	return $fields;
}

public function user_register_allow_email_login($user_id){
	$user = get_userdata($user_id);
	if(!$user->username){
		add_user_meta($user_id, 'username', $user->user_email, true);
	}
}

아래의 코드를 찾아서

foreach($wpmem_fields as $meta_key=>$field){
	if($field['required'] && !in_array($meta_key, array('password', 'confirm_password', 'password_confirm'))){
		if($meta_key == 'confirm_email') $meta_key = 'user_email';
		if(!$current_user->{$meta_key}){
			echo '<script>alert("'.__('Please enter the required information.', 'cosmosfarm-members').'");</script>';
			echo "<script>window.location.href='{$profile_url}';</script>";
			return;
		}
	}
}

아래의 코드로 교체해보시겠어요?

$meta_arr = $option->allow_email_login ? array('username', 'password', 'confirm_password', 'password_confirm') : array('password', 'confirm_password', 'password_confirm');
foreach($wpmem_fields as $meta_key=>$field){
	if($field['required'] && !in_array($meta_key, $meta_arr)){
		if($meta_key == 'confirm_email') $meta_key = 'user_email';
		if(!$current_user->{$meta_key}){
			echo '<script>alert("'.__('Please enter the required information.', 'cosmosfarm-members').'");</script>';
			echo "<script>window.location.href='{$profile_url}';</script>";
			return;
		}
	}
}

빠른 시일 내에 업데이트에 반영하도록 하겠습니다.

고맙습니다.