I was also getting this error and it appears the serverUrl is case sensitive? I had the server Url entered in my JSON file as "https://MYSERVER" but when I looked at how the URL is displayed in the browser window it was all lower case, so I modified the server URL in the JSON file to "https://myserver" and it worked!
I tried this, but it did not work.
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i solved the problem with this:
Not "contentUrl": "Default" but "contentUrl": ""
Hope it works for you too.
Thank you very much