What's the best path for Jira Cloud --> Jira Server migration with Zephyr?

I'm trying to migrate from Jira Cloud --> Jira Server migration with Zephyr. It keeps triggering errors. I'm at the point of totally unistalling Zephyr from Jira and reinstalling it, but it just returns this vague error 'Error has occured, check the logs'.

 

Has anyone else done this and do you have any advice on what works, a best practice or even an ok-practice at this point?

 

Also, the license key doesn't work. Is this just Zephyr being Zephyr?

 

Thanks All. and err Grrrrr!