HTTP load failed (error code: 12 [1:12]) - ios

Question

In my app, I try to download a batch of images from a server.
I'm getting a number of errors, one of which I can't find an explanation for. I checked Apple's URL loading system error codes, but I couldn't fine one for error code 12. I also got HTTP load failed (error code: -999) (which is NSURLErrorCancelled) and Task finished with error - code: -1001(which is NSURLErrorTimedOut).
I recently switched from using deprecated NSURLConnection to NSURLSession. With the former, I also got the 999 and 1001 errors, but I'm trying to find out what error code 12 means.
- (void)loadImage:(LeafletURL*)leafletURLInput isThumbnail:(BOOL)isThumbnailInput isBatchDownload:(BOOL)isBatchDownload isRetina:(BOOL)isRetina
{
isRetina_ = isRetina;
if (session)
{
/*is this the right call here? */
[session invalidateAndCancel];
[session release];
session = nil;
}
if (mImageData)
{
[mImageData release];
mImageData = nil;
}
self.leafletURL = leafletURLInput;
self.isThumbnail = isThumbnailInput;
NSString* location = (self.isThumbnail) ?leafletURL.thumbnailLocation :leafletURL.hiResImageLocation;
//// Check if the image needs to be downloaded from server. If it is a batch download, then override the local resources////
if ( ([location isEqualToString:kLocationServer] || (isBatchDownload && [location isEqualToString:kLocationResource])) && self.leafletURL.rawURL != nil )
{
//NSLog(#"final loadimage called server");
//// tell the delegate to get ride of the old image while waiting. ////
if([delegate respondsToSelector:#selector(leafletImageLoaderWillBeginLoadingImage:)])
{
[delegate leafletImageLoaderWillBeginLoadingImage:self];
}
mImageData = [[NSMutableData alloc] init];
/*download tasks have their data written to a local temp file. It’s the responsibility of the completion handler to move the file from its temporary location to a permanent location.*/
NSURL* url = [NSURL URLWithString:[leafletURL pathForImageOnServerUsingThumbnail:self.isThumbnail isRetina:isRetina]];
NSURLRequest* request = [NSURLRequest requestWithURL:url];
session = [NSURLSession sharedSession];
dataTask = [session dataTaskWithRequest:request
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
{
// do something with the data
}];
[dataTask resume];
}
//// if not, tell the delegate that the image is already cached. ////
else
{
if([delegate respondsToSelector:#selector(leafletImageLoaderDidFinishLoadingImage:)])
{
[delegate leafletImageLoaderDidFinishLoadingImage:self];
}
}
}