Cannot find the resource for the request $metadata

Good Evening All

First post so I am hoping you will all help me where you can!

I have inherited a lovely Power BI dashboard which has been working successfully pointing to our pre-production SharePoint 2013 on-prem site.  The developer was a contractor who has since left.  After leaving, the customer signed off on the dashboard and the underlying application on SharePoint.

The application was then copied across to our production SharePoint 2013 on-prem environment using Sharegate.  I now need to change the Power BI report to point to the new site.  This seems super straight forward, so i did the following:

• Edit Queries > Data Source Settings
• Chose the SharePoint datasource and Change Source
• I changed the url from our pre-prod site to the prod site
• Close and Apply

Whilst applying the query changes I get the above error.

"SharePoint: Request failed (404): The remote server returned an error: (404) Not Found. (Cannot find the resource for the request $metadata.)

When i go back and change the datasource to the pre-prod url it is absolutely fine.

Has anyone ever seen this.  It is really delaying to roll out of the application solution to production for the customer.

Thanks in advance - all help is majorly appreciated.

Cassy