iOS App crashes only when attached to the debugger - ios

Question

i have an issue where my app crashes when attached to the debugger in just one case. My App downloads json data and transform it into core data objects. For this purpose i start a background thread to load it and have a successHandler handling the import:
RemoteConnectionService.sharedInstance.sendRequest(UserRequest(userID: user!.webID.integerValue), successHandler: { requestHandler, result in
var userData = result["user"] as! [String: AnyObject]
self.user?.update(userData)
self.tableView.reloadData()
}, errorHandler: nil)
this works everywhere as expected. the successHandler is called on the main thread, core data gets updated and all. but when doing this special request (it happens always and only with this request) Xcode stop pointing to
class AppDelegate: UIResponder, UIApplicationDelegate
with no specific failure reason. here's the backtrace (via bt):
* thread #1: tid = 0x6ab4b, 0x00000001051b19c0 libobjc.A.dylib`objc_release + 16, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
frame #0: 0x00000001051b19c0 libobjc.A.dylib`objc_release + 16
frame #1: 0x00000001051b28cd libobjc.A.dylib`(anonymous namespace)::AutoreleasePoolPage::pop(void*) + 591
frame #2: 0x00000001082348d8 CFNetwork`-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 207
frame #3: 0x0000000108234a37 CFNetwork`-[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 48
frame #4: 0x00000001081043a7 CFNetwork`___ZN27URLConnectionClient_Classic26_delegate_didFinishLoadingEU13block_pointerFvvE_block_invoke + 107
frame #5: 0x00000001081d1891 CFNetwork`___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 273
frame #6: 0x00000001080ef6e6 CFNetwork`RunloopBlockContext::_invoke_block(void const*, void*) + 72
frame #7: 0x00000001057c13d4 CoreFoundation`CFArrayApplyFunction + 68
frame #8: 0x00000001080ef5a7 CFNetwork`RunloopBlockContext::perform() + 133
frame #9: 0x00000001080ef3e6 CFNetwork`MultiplexerSource::perform() + 256
frame #10: 0x00000001080ef1fc CFNetwork`MultiplexerSource::_perform(void*) + 72
frame #11: 0x00000001057ee4b1 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
frame #12: 0x00000001057e437d CoreFoundation`__CFRunLoopDoSources0 + 269
frame #13: 0x00000001057e39b4 CoreFoundation`__CFRunLoopRun + 868
frame #14: 0x00000001057e33e6 CoreFoundation`CFRunLoopRunSpecific + 470
frame #15: 0x000000010abdba3e GraphicsServices`GSEventRunModal + 161
frame #16: 0x00000001063cab60 UIKit`UIApplicationMain + 1282
* frame #17: 0x00000001049556b7 App`main + 135 at AppDelegate.swift:13
frame #18: 0x00000001088fd145 libdyld.dylib`start + 1
What i don't get here is that when trying to profile it in instruments or just running it in the simulator without Xcode it works just fine. On the device it crashes on that particular request no matter what.
device backtrace looks a bit different:
* thread #1: tid = 0x19e6ab, 0x0000000192490174 libobjc.A.dylib`objc_release + 20, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x13723bec8)
frame #0: 0x0000000192490174 libobjc.A.dylib`objc_release + 20
frame #1: 0x0000000192491724 libobjc.A.dylib`(anonymous namespace)::AutoreleasePoolPage::pop(void*) + 564
frame #2: 0x00000001817d2e08 CFNetwork`-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 216
frame #3: 0x00000001817d2f7c CFNetwork`-[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 60
frame #4: 0x00000001816a78e4 CFNetwork`___ZN27URLConnectionClient_Classic26_delegate_didFinishLoadingEU13block_pointerFvvE_block_invoke + 104
frame #5: 0x0000000181770540 CFNetwork`___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 104
frame #6: 0x0000000181693b54 CFNetwork`RunloopBlockContext::_invoke_block(void const*, void*) + 76
frame #7: 0x0000000181c10aac CoreFoundation`CFArrayApplyFunction + 68
frame #8: 0x0000000181693a00 CFNetwork`RunloopBlockContext::perform() + 136
frame #9: 0x00000001816938b4 CFNetwork`MultiplexerSource::perform() + 312
frame #10: 0x00000001816936e0 CFNetwork`MultiplexerSource::_perform(void*) + 68
frame #11: 0x0000000181ce69ec CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24
frame #12: 0x0000000181ce5c90 CoreFoundation`__CFRunLoopDoSources0 + 264
frame #13: 0x0000000181ce3d40 CoreFoundation`__CFRunLoopRun + 712
frame #14: 0x0000000181c110a4 CoreFoundation`CFRunLoopRunSpecific + 396
frame #15: 0x000000018adab5a4 GraphicsServices`GSEventRunModal + 168
frame #16: 0x0000000186542aa4 UIKit`UIApplicationMain + 1488
* frame #17: 0x0000000100069c8c App`main + 164 at AppDelegate.swift:13
frame #18: 0x0000000192ae6a08 libdyld.dylib`start + 4
Any suggestions on how to debug this? Very much appreciated :)