can you please show me the screenshot of the microflow, that you are using to make a GET call?
Also, please share the screenshot of the JSON structure as well.
That looks like you may be receiving HTML or XML, possibly in an error message.
To check this, I would put the log level of the REST Consume node to TRACE. This will log the full request and response in the logs so you should be able to see what is being returned and what is causing the error.
I hope this helps.
Can you please try without adding application/json in the custom headers. Have you added the response as apply import mapping and given your import mapping?If didn't work please put a log message under call REST and check the errors and also please share those screenshot.