We have a Problem with JSON here:
Either we got no JSON from the API. Or the basenode-parameter is not ok.
Switch on the Debugmode of the Plugin!
Switch to gutenberg-Blocks!
There the JSON Content Importer Block gives an easier way to use the JSON-APIs.
Or: Switch on the Debugmode by adding "debugmode=10" in the Shortcode.

If all is without success: Open ticket at wordpress.org please

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