I am getting this error on Switch, iOS and Windows 10. Using Win10 is not a workaround.
Especially because the whole reason I got the realm was to upload my world from my Switch to play on Windows with a dedicated server...
Getting the exact issue, on Switch, iOS, and Windows 10.
In response to Gabriela, this is a fluke and only uploads the world seed. Any progress made in the world will not be uploaded.