Have contacted support via the link inside the error but haven't heard a response in nearly a week.
When accessing the Gannt Cloud plugin from any of our Jira projects:
Backend Error
An unknown error occured while contacting with Add-on's backend: 500 Internal Server Error
The problem might be temporary so you can try to reload this page later. Report a problem
Our account is under hrmweb.jira.com, please assist.
Regards,
Mick Alexander