The contents of the document folder - the file cannot be opened because there is no such file

Context: Xcode 8.3.2 / Swift 3

I am uploading a bunch of MP3s to a sub folder inside the Documents folder. Then I confirm success:

• try FileManager.default.contentsOfDirectory(atPath: folderPath)
  
   I can see my files are listed
• using Xcode I am doing Window - Devices - myDevice - myApp - downloadContainer. I get a copy of the sandbox on my computer desktop. I go to the corresponding subfolder in the Documents folder and of course my MP3 files are there.
• last but not least ... in my mobile app, I simply select one of the downloaded MP3 files and hit play. He plays.

Then I restart the application. No saved song will play for the above reason: The "name_here" file cannot be opened because there is no such file. FileManager.default.contentsOfDirectory () still lists them there; when i check the sandbox they are all there.

I have no idea. Ideas, anyone?

PS: the error occurs when I

try Data.init(contentsOf: URL(fileURLWithPath: file_path_here))

      

        
        
        
      

    

I usually didn't need to load stuff into memory; AVPlayer plays them just fine from disc; however, I have to decrypt them first, so I fill "NSData" with the contents of any file. It is useless to say that nothing reaches my decoder because ... The file cannot be opened because there is no such file

Why does the same code work the first time (when I load it) but fail in a later session ???