Uncaught Error: Class ‘PHPMailer\\PHP

Hi,

After I upgrade to WP v 5.5.1 suddenly all my form that supose to send an email to me stop working. There is error in error log that said Got error ‘PHP message: PHP Fatal error: Uncaught Error: Class ‘PHPMailer\\PHP

 AH01071: Got error 'PHP message: PHP Fatal error:  Uncaught Error: Class 'PHPMailer\\PHPMailer\\SMTP' not found in /var/www/wp-includes/PHPMailer/PHPMailer.php:1821\nStack trace:\n#0 /var/www/wp-includes/PHPMailer/PHPMailer.php(1942): PHPMailer\\PHPMailer\\PHPMailer->getSMTPInstance()\n#1 /var/www/wp-includes/PHPMailer/PHPMailer.php(1858): PHPMailer\\PHPMailer\\PHPMailer->smtpConnect(Array)\n#2 /var/www/wp-includes/PHPMailer/PHPMailer.php(1601): PHPMailer\\PHPMailer\\PHPMailer->smtpSend('Date: Thu, 24 S...', 'This is a multi...')\n#3 /var/www/wp-includes/PHPMailer/PHPMailer.php(1437): PHPMailer\\PHPMailer\\PHPMailer->postSend()\n#4 /var/www/wp-content/plugins/formcraft3/formcraft-main.php(2235): PHPMailer\\PHPMailer\\PHPMailer->send()\n#5 /var/www/wp-includes/class-wp-hook.php(287): formcraft3_test_email('')\n...'

Can you give me whats going on?