Icingaweb2 2.8.1 is out for a couple of weeks now and adds support for PHP 7.4.
A commit references this bug: Author: lme Date: Mon Aug 10 20:21:30 UTC 2020 New revision: 544645 URL: https://svnweb.freebsd.org/changeset/ports/544645 Log: net-mgmt/icingaweb2: - Update to 2.8.1 - Pet portfmt PR: 248310 Submitted by: Jonas Palm <jonaspalm@posteo.de> Changes: head/net-mgmt/icingaweb2/Makefile head/net-mgmt/icingaweb2/distinfo head/net-mgmt/icingaweb2/files/patch-library_Icinga_Web_View.php head/net-mgmt/icingaweb2/files/patch-library_vendor_Zend_Registry.php head/net-mgmt/icingaweb2/files/patch-library_vendor_lessphp_lessc.inc.php head/net-mgmt/icingaweb2/pkg-plist
I updated the port to 2.8.1