Hi wwinfried5p, welcome to Thunkable! 
Be sure to check out How to ask Great Questions v2.0, the Community Guidelines, and our Getting Started Guide to make the best of your Thunkable Community experience!
Are you testing the API call on the web preview? If so, the 404 error might just be a CORS issue. CORS can prevent a web API from working on some devices or browsers.
You can check your browser console to see if this is the case. If so, you’ll see an error like this:
“Access to fetch at [URL] from origin ‘https://x.thunkable.com’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.”
You can find more information here: CORS Policy | Thunkable Docs