Your server is running PHP 7.4.1. PHP 7.4 is not tested with CB. CB is tested working up to PHP 7.3.9. It's possible your issue is due to PHP version. The error doesn't make any sense unless some PHP behavior has changed, something is altering the POST data, or our PHPMail library is not compatible with PHP 7.4. Please downgrade your PHP to a supported version and retry. Have added a feature ticket for PHP 7.4 compatibility testing and fixing.
forge.joomlapolis.com/issues/7811