AVAssetExportSession Error -11820

I'm writing an application that works with video using AVFoundation.

The behaviour of my application is simple: I take a video from the camera roll, then I create an AVMutableComposition with some audio tracks. With the mix composition i initialize an AVAssetExportSession that stores the video file in the documents directory of my app.

Until this point everything it's ok: my video is stored and I'm able to play it in another controller. If I take the video that i have just stored in my documents folder to make some editing (in the same way of the first time AVmutableComposition, AVAssetExportSession) it's ok again.

But the third time I do this process to editing a video the AVAssetExportSession status becomes "Fail" and with this error:

"Domain=AVFoundationErrorDomain Code=-11820 "Cannot Complete Export" UserInfo=0x1a9260 {NSLocalizedRecoverySuggestion=Try exporting again., NSLocalizedDescription=Cannot Complete Export}"

I have read that is a general error where the session couldn't be exported. What is the sense of this? Why only the third time that i made the editing process? Could it be a memory management mistake? A bug?. This is the code of my AVAssetExportSession:

 _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];   
_assetExport.shouldOptimizeForNetworkUse = YES;

///data odierna
NSDateFormatter *format = [[NSDateFormatter alloc] init];
[format setDateFormat:@"ddMMyyyyHHmmss"];

NSDate *now = [[NSDate alloc] init];

NSString *dateString = [format stringFromDate:now];
[now release];
[format release];
NSString* ext = @".MOV";
NSString* videoName=[NSString stringWithFormat:@"%@%@", dateString, ext];

///data odierna
NSString *exportPath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:videoName];

if ([[NSFileMan