You might have seen a loading image in the footer of your WHMCS admin page. If you inspect the page, you’ll see it’s got some tags for the lightbox.
The issue is related to Cloudflare Rocket Loader, you can simply create a page rule to disable Rocket Loader on the admin pages or disable Rocket Loader altogether.
Disable Rocket Loader with Cloudflare Page Rule
If you wish to disable Rocket Loader for a specific URL then you can use Cloudflare Page Rules using the following configuration.