Hello,
this is not a valid JSON string, therefore JSONSlurper throws an error here. For some reason, it is a mix of HTML and JSON format. To parse only JSON part:
String response = "<html><body>{‘auth_code’: ‘1xxxxxxxxxxxxxxxxxxxxxxx’}</body></html>"
String jsonPart = response.substring(response.indexOf("{"), response.lastIndexOf("}") + 1)
And then you can use JSONSlurper.
def jsonSlurper = new JsonSlurper()
def jsonResponse = jsonSlurper.parseText(jsonPart)