title: AVAssetExportSession failed with unknown error -12780 for specific video
body: <p>I have a problem tracing the underlying issue behind my asset export session failure. The issue is for one video only, and I believe the problem is in its audio track, since I successfully exported the asset without the audio track (only the video track).</p>
  
  <p>The video track is decoded with AVAssetReader and the sample buffers are processed before being rewritten into a new video track; the audio track is passed with no decoding nor any intermediate processing. However, even without processing the video sample buffers, the same failure occurred.</p>
  
  <p>I also tried doing it the other way round--with audio only and no video track--and still other videos worked just fine and this particular video failed. I suppose there's an inherent problem with the video's audio track, but I can't infer what the problem is, and hence I can't tackle it. Here's my code:</p>
  
  <pre><code>AVAssetExportSession* assetExport = [[AVAssetExportSession alloc] initWithAsset:composition
                                                                        presetName:AVAssetExportPresetHighestQuality];
  
  assetExport.outputFileType = @"com.apple.quicktime-movie";
  assetExport.outputURL = [NSURL fileURLWithPath:path];
  
  __weak typeof(self) weakSelf = self;
  [assetExport exportAsynchronouslyWithCompletionHandler:^{
  
      switch (assetExport.status) {
          case AVAssetExportSessionStatusCompleted: NSLog(@"Asset combined");
              break;
          case AVAssetExportSessionStatusFailed: NSLog(@"Asset combination failed");
              break;
          default: NSLog(@"Asset combination completed with unknown status: %@", @(assetExport.status));
              break;
      }
  }];
  </code></pre>
  
  <p>The problem is supposed to be in the asset export session; track insertion to the AVMutableComposition worked just fine. Here's the error message of the AVAssetExportSession:</p>
  
  <pre><code>Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed"
  UserInfo={NSUnderlyingError=0x6040001338d0 {Error Domain=NSOSStatusErrorDomain Code=-12780 "(null)"}, 
  NSLocalizedFailureReason=An unknown error occurred (-12780), NSLocalizedDescription=The operation could not be completed}
  </code></pre>