SIGSEGV Crash on iOS TestFlight

I have a weird crash that only happens on special TestFlight distributions. I know what causes the crash (a simple while loop, see below), but I don’t understand why it does not crash on my simulator, neither on the iPhone nor on the iPad (with a development profile).

I am constantly distributing adhoc builds to track the failure, but I finally gave up. How can I track this error more efficiently?

Here's the TestFlight crash report:

    Exception reason

SIGSEGV

Stacktrace

PRIMARY THREAD THREAD 0

0 Meernotes 0x0019649a testflight_backtrace + 158
1 Meernotes 0x001970c4 TFSignalHandler + 244
2 libsystem_c.dylib 0x32d9d7ec _sigtramp + 48
3 libobjc.A.dylib 0x37d34eac _ZN12_GLOBAL__N_119AutoreleasePoolPage3popEPv + 224
4 libobjc.A.dylib 0x37d34eac _ZN12_GLOBAL__N_119AutoreleasePoolPage3popEPv + 224
5 libobjc.A.dylib 0x37d34dc8 _objc_autoreleasePoolPop + 12
6 CoreFoundation 0x358e1cfe _CFAutoreleasePoolPop + 18
7 UIKit 0x333d5c92 _wrapRunLoopWithAutoreleasePoolHandler + 42
8 CoreFoundation 0x35965b1a __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 18
9 CoreFoundation 0x35963d56 __CFRunLoopDoObservers + 258
10 CoreFoundation 0x359640b0 __CFRunLoopRun + 760
11 CoreFoundation 0x358e74a4 CFRunLoopRunSpecific + 300
12 CoreFoundation 0x358e736c CFRunLoopRunInMode + 104
13 GraphicsServices 0x37583438 GSEventRunModal + 136
14 UIKit 0x333f3cd4 UIApplicationMain + 1080
15 Meernotes 0x000850da main (main.m:16)
16 Meernotes 0x0008508f start + 39

And here is the code that runs it (but this is normal on the simulator / iphone):

        while (textStillFits) {

        textToBeKeptBeforeCutting = textToBeKept;
        textToBeMovedBeforeCutting = textToBeMoved;

        rangeCutOffPosition = [settings determineFirstCutOffRangeOfString:textToBeMoved];
        textToBeKeptTemp = [textToBeMoved substringToIndex:rangeCutOffPosition.location];
        textToBeMovedTemp = [textToBeMoved substringFromIndex:rangeCutOffPosition.location];
        textToBeMoved = textToBeMovedTemp;
        textToBeKept = [NSString stringWithFormat:@"%@%@", textToBeKept, textToBeKeptTemp];
        NSUInteger l = [settings linesOfText:textToBeKept];
        if (l > maxLines) {
            textStillFits = NO;
            textToBeKept = textToBeKeptBeforeCutting;
            textToBeMoved = textToBeMovedBeforeCutting;
            // brakes here!
        }else {
            textStillFits = YES;
        }
    }

I looked at this, but that did not help: ad hoc iphone sigsegv crash

Another context, as @ott mentioned that NSNotFound can cause this:

    -(NSRange)determineFirstCutOffRangeOfString:(NSString *)s {

    NSRange rangeSpace = [s rangeOfString:@" " options:NSCaseInsensitiveSearch];
    NSRange rangeReturn = [s rangeOfString:@"\n" options:NSCaseInsensitiveSearch];
    if (rangeSpace.location == NSNotFound && rangeReturn.location == NSNotFound) {
        rangeSpace = NSMakeRange(1, 0); // don't take 0,0 as this will lead to endless loop
        rangeReturn = NSMakeRange(1, 0);
        NSLog(@"NEITHER SPACE NOR RETURN FOUND");
    }

    NSRange rangeCutOffPosition = NSMakeRange(0, 0);
    if (rangeSpace.location < rangeReturn.location) {
        // i.e. if the space char is the lowest char where you can cut off
        rangeCutOffPosition = NSMakeRange(rangeSpace.location+1,0); // +1, i.e. always cut off AFTER the space
    }  else {
        // i.e. if the return char is the lowest char where you can cut off
        rangeCutOffPosition = NSMakeRange(rangeReturn.location+1,0); // +1, i.e. always cut off AFTER the return
    }

    return rangeCutOffPosition;
}