Thanks all for contributing to this thread, appreciated.
I can confirm that this issue does relate to the Cloudflare Rocket Loader service.
I’ve checked out a couple of cases on our desk and disabling this service for your WHMCS installation looks to resolve this in every single case.
You can identify if Rocket Loader is enabled by viewing the Page Source in your browser when navigating to your WHMCS installation then search for the line:
This will be located in the footer area and gives you an indication that this service is indeed enabled for your WHMCS installation.
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