After upgrading to EasyApache4 and then running a Softaculous upgrade of WHMCS there are a couple of things you need to do to ensure WHMCS functions as expected.
Ensure you are using the latest version of PHP (v7) and enough memory is allocated to PHP. See our post here
Ensure you have the Ioncube loader for PHP 7 installed
In WHM go to Software -> EasyApache4 -> Customize -> PHP Extensions
Search for php70-php-ioncube6 and enable it -> Click next
You will then move onto the review screen at which point click on the “Provision” button on the bottom left of that screen.