Yup, I got a 500 internal server error after updating to PHP 7.4 yesterday. Then I checked this page https://docs.whmcs.com/System_Environment_Guide#PHP and saw that the latest active WHMCS version doesn't support it yet. I read that PHP 7.4 came out back in Nov 2019 so hopefully we can get WHMCS version 8 out soon.
@brian! That explanation makes sense