A random crash on NSDateFormater

My App keep getting NSDateFormatter crash randomly when I check iConnect on AppleStore, I did a lot search regarding of NSDataFormatter crash, mostly the reason is memory leak,

However, For me, it looks like Thread safe related,

here is stack

Thread 1 name:

Thread 1 Crashed:

0 libdispatch.dylib 0x0000000184d07b58 _dispatch_semaphore_dispose$VARIANT$mp + 76 (semaphore.c:69)

1 libdispatch.dylib 0x0000000184d06c7c _dispatch_dispose$VARIANT$mp + 80 (object.c:222)

2 Foundation 0x0000000185cdf7d0 -[NSDateFormatter dealloc] + 76 (NSDateFormatter.m:291)

3 App 0x0000000100e1a3d4 +[WICPage dateFormatter] (WICPage.m:80)

4 App 0x0000000100da6014 -[PagerSettings setServerTime:] (PagerSettings.m:2271)

5 App 0x0000000100db0f4c -[WICDownLinkPDU parseRawData:] (WICDownLinkPDU.m:368)

6 App 0x0000000100db06b8 -[WICDownLinkPDU initWithRawData:] (WICDownLinkPDU.m:254)

7 App 0x0000000100db0598 +[WICDownLinkPDU pduWithData:] (WICDownLinkPDU.m:237)

8 App 0x0000000100d89d28 -[ConnectivityRobot updateDataFromServer] (ConnectivityRobot.m:1040)

9 App 0x0000000100d895dc -[ConnectivityRobot checkDataFromServer] (ConnectivityRobot.m:945)

10 App 0x0000000100fc6ffc __46-[VACallManager vomopingRequestFromSIPOption:]_block_invoke_4 (VACallManager.m:150)

11 libdispatch.dylib 0x0000000184d05088 _dispatch_call_block_and_release + 24 (init.c:994)

12 libdispatch.dylib 0x0000000184d05048 _dispatch_client_callout + 16 (object.m:502)

13 libdispatch.dylib 0x0000000184d12378 _dispatch_root_queue_drain + 1028 (inline_internal.h:2500)

14 libdispatch.dylib 0x0000000184d11f10 _dispatch_worker_thread3 + 120 (queue.c:6104)

15 libsystem_pthread.dylib 0x0000000184fab120 _pthread_wqthread + 1268 (pthread.c:2286)

16 libsystem_pthread.dylib 0x0000000184faac20 start_wqthread + 4

the instance of creation NSDateFormatter

static NSDateFormatter *pageDateFormatter=nil;
+ (NSDateFormatter *) dateFormatter
{
    static dispatch_once_t onceToken = 0;
    dispatch_once(&onceToken, ^{
        /
            pageDateFormatter = [[NSDateFormatter alloc] init];
            [pageDateFormatter setDateFormat:@"yyyy/MM/dd HH:mm:ss zzzz"];
        /
    });
    return pageDateFormatter;
}

and that function was called in GCD to retrive some information from server with Date and timestamp on server side.

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
...
[WICPage dateFormatter];
...
    });

Any thought on this crash?

Thanks,