This doesn't seem to be the template we provide for Salesforce integration, as you're sending requests to test.salesforce.com instead of login.salesforce.com. Did you make any change to the default template to target a testing sandbox?
The problem is that Salesforce is returning an HTML page, when the web service should return a JSON response. Probably this HTML page contains some error description, but we can't see that from the screenshot, you will need to scroll down to see it... That error description will give you some pointers...