Undefined constant "PHPMailer\PHPMailer\FILTER_FLAG_HOST_REQUIRED"

Getting an unusual error when attempting to submit the contact form on the following page which uses WebAssist Send Email behaviour:
https://www.southwestpropertycentre.co.uk/property-sales-lets-enquiries.php?c=1

Fatal error: Uncaught Error: Undefined constant "PHPMailer\PHPMailer\FILTER_FLAG_HOST_REQUIRED" in /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php:3632 Stack trace: #0 /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php(3598): PHPMailer\PHPMailer\PHPMailer::isValidHost('www.southwestpr...') #1 /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php(2331): PHPMailer\PHPMailer\PHPMailer->serverHostname() #2 /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php(1427): PHPMailer\PHPMailer\PHPMailer->createHeader() #3 /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php(1320): PHPMailer\PHPMailer\PHPMailer->preSend() #4 /home/storage/824/4137824/user/htdocs/webassist/email/WA_Email.php(477): PHPMailer\PHPMailer\PHPMailer->send() #5 /home/storage/824/4137824/user/htdocs/thankyou.php(57): WA_Email->send(0) #6 {main} thrown in /home/storage/824/4137824/user/htdocs/webassist/email/phpmailer/src/PHPMailer.php on line 3632

This form was working, previously, and we haven't changed anything so is this a PHP change? The server is Linux and PHP version is 8.0.
Hope you can help. Thank you.
Regards
NJ

I had to update the phpmailer folder and WA_Email.php file when upgrading my sites to PHP8, so you could try replacing the file and folder with the attached version which works for me (make a copy of yours before replacing though, just in case you need to roll back!)