Loading UIImage from disk sometimes fails with permission error in background state?

I have this strange issue that I am having trouble resolving. I am creating an App which allows music to be played back. When the screen is locked (and there is a currently playing song), the lock screen will populate with a bunch of data. One piece is the album art.

The problem is that after the phone is locked and I skip a few tracks (forwards or backwards), the UIImages are no longer being loaded. If I test out the functionality and quickly skip forward and backwards in my playback queue, the album art will appear for the first 4-5 songs. After that, the images stop appearing because I get a NSFileReadNoPermissionError from my code that grabs the image. I understand that I apparently do not have permission to access the png image files, but I do not understand why. My application created them, saved them on disk, and is now trying to load them from disk while my app is running in a background state.

The relevant code snippet:

+ (void)updateLockScreenInfoAndArtForSong:(Song *)song
{

    NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    NSString *artDirPath = [documentsPath stringByAppendingPathComponent:@"Album Art"];
    NSString *path = artDirPath;
    //-----> LIST ALL FILES for debugging <-----//
    NSLog(@"LISTING ALL FILES FOUND");

    int count;
    NSArray *directoryContent = [[NSFileManager defaultManager] contentsOfDirectoryAtPath:path
                                                                                    error:NULL];
    for (count = 0; count < (int)[directoryContent count]; count++)
    {
        NSLog(@"File %d: %@", (count + 1), [directoryContent objectAtIndex:count]);
    }
    //-----> END DEBUG CODE <-----//


    Song *nowPlayingSong = [MusicPlaybackController nowPlayingSong];

    Class playingInfoCenter = NSClassFromString(@"MPNowPlayingInfoCenter");
    if (playingInfoCenter) {
        NSMutableDictionary *songInfo = [[NSMutableDictionary alloc] init];

        NSError *error;
        NSData *data = [NSData dataWithContentsOfURL:
                        [AlbumArtUtilities albumArtFileNameToNSURL:nowPlayingSong.albumArtFileName] options:NSDataReadingUncached error:&error];
        NSInteger code = error.code;
        NSLog(@"Error code: %li", (long)code);    //prints 257 sometimes, which is NSFileReadNoPermissionError

        UIImage *albumArtImage = [UIImage imageWithData:data];

        if(albumArtImage == nil){  //song has no album art, check if its album does
            Album *songsAlbum = song.album;
            if(songsAlbum){
                albumArtImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:
                                                        [AlbumArtUtilities albumArtFileNameToNSURL:songsAlbum.albumArtFileName]]];
            }
        }

        [songInfo setObject:nowPlayingSong.songName forKey:MPMediaItemPropertyTitle];
        NSInteger duration = [nowPlayingSong.duration integerValue];
        [songInfo setObject:[NSNumber numberWithInteger:duration]
                     forKey:MPMediaItemPropertyPlaybackDuration];
        [[MPNowPlayingInfoCenter defaultCenter] setNowPlayingInfo:songInfo];
    }
}

Any help would be immensely appreciated! I have tried so many thing that I am at a loss for what to even try next. Note the above code snippet is called in

- (void)remoteControlReceivedWithEvent:(UIEvent *)event

when event.subtype is UIEventSubtypeRemoteControlNextTrack or UIEventSubtypeRemoteControlPreviousTrack.