Today I've got an exception in my Provider-Hosted App. The message was "File not found" and nothing more...
But I do not work with files, document libraries, attachments or any other resources that can be received as a file in that app. That why such a message was very unexpected.
After debugging I understand where was a problem: I was trying to open Web with absolute Url, not relative.
So, if you have the same exception check your Urls.
Have fun!