Fatal error when sending email with WordPress 5.5.1

Hi
I updated to WordPress 5.5.1 and Newsletter 4.6.19 and I can’t send email anymore.
I got this error :

Fatal error: Uncaught Error: Class ‘PHPMailer\PHPMailer\SMTP’ not found
in /home/codebio/sites/caminarem/wp-includes/PHPMailer/PHPMailer.php on line 1821

Call stack:

PHPMailer\P\PHPMailer::getSMTPInstance()
wp-includes/PHPMailer/PHPMailer.php:1942
PHPMailer\P\PHPMailer::smtpConnect()
wp-includes/PHPMailer/PHPMailer.php:1858
PHPMailer\P\PHPMailer::smtpSend()
wp-includes/PHPMailer/PHPMailer.php:1601
PHPMailer\P\PHPMailer::postSend()
wp-includes/PHPMailer/PHPMailer.php:1437
PHPMailer\P\PHPMailer::send()
wp-content/plugins/newsletters-lite/wp-mailinglist-plugin.php:9366
wpMailPlugin::execute_mail()
wp-content/plugins/newsletters-lite/wp-mailinglist.php:5350
wpMail::admin_send()
wp-includes/class-wp-hook.php:287
WP_Hook::apply_filters()
wp-includes/class-wp-hook.php:311
WP_Hook::do_action()
wp-includes/plugin.php:478
do_action()
wp-admin/admin.php:259

Any clue ?

Thanks a lot