"Connection test failed: Unhandled exception caught in RESTConnector.Connect: Object reference not set to an instance of an object." Error what to do?

We are getting a this error when we try to connect wit a REST API. In Swagger the API shows everything good without warnings. 


We try to make a minimal JSON file only for a FIND action and that works! Does anyone know what the problem can be? see attachment for the JSON file. Thanks in Advance!

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