iPhone app crashes on some devices upon startup, but runs on others - ios

Question

We migrated an iOS 6 app to iOS 7 without altering anything in particular beyond some minor adjustments. We tested the app on the 3, 4, 4S and 5S iPhones and reported no problem.
Now two users are reporting that the app crashes upon startup. I advised one of them to remove the app altogether and install again which resulted in the app running smoothly.
He forwarded the crash log to me but it's a little cryptic. Can anyone help me pinpoint the issue?
EDIT: Symbolicated crash log
Incident Identifier: 687B278C-313F-4533-81E8-0003428A3362
CrashReporter Key: b25d61923c16e6a67af8ea7bf81643ba93f4d1ea
Hardware Model: iPhone4,1
Process: RunControl [3488]
Path: /var/mobile/Applications/DB78DB79-2AB2-4ADD-89C8-BACDBB6BCCF3/RunControl.app/RunControl
Identifier: com.gsddl.RunControl
Version: 1.5 (1.5)
Code Type: ARM (Native)
Parent Process: launchd [1]
Date/Time: 2014-03-10 09:49:34.065 +0100
OS Version: iOS 7.0.6 (11B651)
Report Version: 104
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Triggered by Thread: 3
Last Exception Backtrace:
0 CoreFoundation 0x2dbbbf46 __exceptionPreprocess + 126
1 libobjc.A.dylib 0x37f4b6aa objc_exception_throw + 34
2 CoreFoundation 0x2dbbbe88 +[NSException raise:format:] + 100
3 Foundation 0x2e4dd448 -[NSString stringByAppendingString:] + 84
4 RunControl 0x000b2302 -[RunControlAppDelegate downloadData] (RunControlAppDelegate.m:297)
5 Foundation 0x2e59fc82 __NSThread__main__ + 1058
6 libsystem_pthread.dylib 0x38572c18 _pthread_body + 136
7 libsystem_pthread.dylib 0x38572b8a _pthread_start + 98
8 libsystem_pthread.dylib 0x38570c8c thread_start + 4
Thread 0:
0 libsystem_kernel.dylib 0x38509014 0x384f7000 + 73748
1 libsystem_pthread.dylib 0x38572a74 pthread_create + 156
2 Foundation 0x2e50fb00 -[NSThread start] + 284
3 RunControl 0x000b20c4 -[RunControlAppDelegate initData] (RunControlAppDelegate.m:274)
4 RunControl 0x000b0f58 -[RunControlAppDelegate application:didFinishLaunchingWithOptions:] (RunControlAppDelegate.m:32)
5 UIKit 0x3039f2fa -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 270
6 UIKit 0x3039ed4a -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1386
7 UIKit 0x3039934e -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 710
8 UIKit 0x3033441a -[UIApplication handleEvent:withNewEvent:] + 3126
9 UIKit 0x3033371c -[UIApplication sendEvent:] + 68
10 UIKit 0x30398b38 _UIApplicationHandleEvent + 660
11 GraphicsServices 0x327cf70a _PurpleEventCallback + 606
12 GraphicsServices 0x327cf2f2 PurpleEventCallback + 30
13 CoreFoundation 0x2db869dc __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 32
14 CoreFoundation 0x2db86976 __CFRunLoopDoSource1 + 342
15 CoreFoundation 0x2db8514a __CFRunLoopRun + 1394
16 CoreFoundation 0x2daefc22 CFRunLoopRunSpecific + 518
17 CoreFoundation 0x2daefa06 CFRunLoopRunInMode + 102
18 UIKit 0x30397dd4 -[UIApplication _run] + 756
19 UIKit 0x30393044 UIApplicationMain + 1132
20 RunControl 0x000b0f1a main (main.m:16)
21 RunControl 0x000b0edc start + 36
Thread 1:
0 libsystem_kernel.dylib 0x384f783c 0x384f7000 + 2108
1 libdispatch.dylib 0x38442e08 _dispatch_mgr_invoke + 228
2 libdispatch.dylib 0x3843239e _dispatch_mgr_thread$VARIANT$up + 34
Thread 2:
0 libsystem_kernel.dylib 0x3850ac7c 0x384f7000 + 81020
1 libsystem_pthread.dylib 0x38570dc6 _pthread_wqthread + 306
2 libsystem_pthread.dylib 0x38570c80 start_wqthread + 4
Thread 3 Crashed:
0 libsystem_kernel.dylib 0x3850a1fc 0x384f7000 + 78332
1 libsystem_pthread.dylib 0x38573a2e pthread_kill + 54
2 libsystem_c.dylib 0x384baff8 abort + 72
3 libc++abi.dylib 0x377e9cd2 abort_message + 70
4 libc++abi.dylib 0x378026e0 default_terminate_handler() + 248
5 libobjc.A.dylib 0x37f4b91e _objc_terminate() + 190
6 libc++abi.dylib 0x378001c4 std::__terminate(void (*)()) + 76
7 libc++abi.dylib 0x377ffa18 __cxa_throw + 112
8 libobjc.A.dylib 0x37f4b77e objc_exception_throw + 246
9 CoreFoundation 0x2dbbbe88 +[NSException raise:format:] + 100
10 Foundation 0x2e4dd448 -[NSString stringByAppendingString:] + 84
11 RunControl 0x000b2302 -[RunControlAppDelegate downloadData] (RunControlAppDelegate.m:297)
12 Foundation 0x2e59fc82 __NSThread__main__ + 1058
13 libsystem_pthread.dylib 0x38572c1a _pthread_body + 138
14 libsystem_pthread.dylib 0x38572b8a _pthread_start + 98
15 libsystem_pthread.dylib 0x38570c8c thread_start + 4
Thread 3 crashed with ARM Thread State (32-bit):
r0: 0x00000000 r1: 0x00000000 r2: 0x00000000 r3: 0x00002060
r4: 0x00000006 r5: 0x026f1000 r6: 0x00000000 r7: 0x026effb4
r8: 0x16dbd9d0 r9: 0x00000001 r10: 0x16d52ce0 r11: 0x00000000
ip: 0x00000148 sp: 0x026effa8 lr: 0x38573a33 pc: 0x3850a1fc
cpsr: 0x00000010
And RunControlAppDelegate.m (around 297):
[295] - (void) downloadData {
[296] NSString *stringURL = [config valueForKey:#"Users"];
[297] NSURL *url = [NSURL URLWithString:[self.URL stringByAppendingString:stringURL]];
[298] NSData *urlData = [NSData dataWithContentsOfURL:url];
...

Answer 1

I would venture to guess your update included 64 bit update iOS7 + iPhone5 and your data might be formatted with incorrect formatter:
ie: [NSString stringWithFormat:#"Some foo %d"];
This might work with 32 bit int but it will fail on a 64 bit platform.
Are there any warnings when you compile your binary?

Answer 2

The reference to stringURL was indeed nil which resulted in the crash. This was due to a silly race condition.