I was getting a huge number of files listed by Defender as "Unknown file in WordPress Core" so I tried to run a conflict test and run a new Defender scan when having only Defender and the WPMU DEV dashboard plugins active on the site. But the Defender scan was taking too long so I decided to cancel it and try creating a staging site for testing purposes using Shipper.
And then I got this error when trying to run Shipper (both when exporting or migrating and the same issue appeared on 2 newly created staging sites):
"Error talking to info-get, the service responded with 400: The provided domain and api_key combination are not registered. Please install the WPMU DEV Dashboard plugin.
Service error: status: 400;
No remote data to process"
But the WPMU DEV Dashboard was already installed on both the original site and the destination site.