That screenshot is an error within your app, and is about as generic as an error can get.

In order to be able to get past this you are going to need to get more information.
A couple of suggestions that may help you if you haven't already tried them:

• Enable developer debugging on your test site.
• Attempt to access the file in the app.
• Check the access logs for your webserver and identify one of the failed requests.
  If there are no requests, then why is the app not making the requests, if you find the requests note the date/time.
  
• Check the error logs for your webserver looking at the date/time you found in the request, check around that time in general, are there any errors that have been logged?
• Get further information out of the app (how you do this is down to your app, we can't help here) ideally you will want to find the information sent to the server (headers + payload) and the response information.

Hopefully there will be something obvious there.

Cheers
Sam