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.
Step 1
Ensure you are using the latest version of PHP (v7) and enough memory is allocated to PHP. See our post here
Step 2
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
Step 3
You will then move onto the review screen at which point click on the “Provision” button on the bottom left of that screen.