I believe your problem is in the web service call URL, can you test that directly in the browser?
Looking at a custom list in SharePoint here I can see that the custom columns of the User type (People Picker) fields show as "UserId" only and not as an expandable field as you have it in your query string, so perhaps your URL should look like:
/_api/web/lists/GetByTitle('list name')/items?$select=[select fields]&$filter=UserId eq '[id]'
Either way if you test your url with a valid SP User ID in any browser you should get the expected response.
Finally you can also check the variables you are passing, ensure that [%Current Item:User%] is definitely just the SP User Id (e.g. an int like 10).
Hope that helps,