Only PHP version 7.2 is supported.
When using the php 7.2 we are hitting on this error when using a defined form.
Your form could not be submitted for the following reason(s):
Warning: [ err 2/line 277/mailer.cls.php] Use of undefined constant writeErrorLog - assumed 'writeErrorLog' (this will throw an Error in a future version of PHP)
And does WFB 2.9 Build 5519 fully supports PHP 7.2?