In case that can help here are the exact traces I'm seeing.
Note that I made a test with a fake key (not valid), and I get the same error, but systematically (when I relaunch the app, it shows the same popup + crash), which makes sense.
When I'm using the valid key, I see this error only from time to time (and usually the first time I'm launching the app after a while (a day?)). If I'm relaunching the app just after the error, it always work. It looks to me like there's a cache involved in this error, maybe something is cached the first time the app crashes, and reused on the second launch, which allows the app to work this time. What I don't understand is that it looks like I'm the only one to have this problem.
Is there a way to turn on more debug traces on the SDK? I'm ready to help as much as I can, I can install debug SDK, or whatever is necesary to understand what's happening.
2015-06-18 10:12:21.066 JeuDePiste[2412:694523] Flurry: Starting session on Agent Version [Flurry_iOS_142_6.0.0]
2015-06-18 10:12:29.379 JeuDePiste[2412:694523] initQCARParameters start
2015-06-18 10:12:29.380 JeuDePiste[2412:694523] initQCARParameters end
2015-06-18 10:12:29.388 JeuDePiste[2412:694523] ARParentVC: creating
2015-06-18 10:12:29.389 JeuDePiste[2412:694523] ARVC: loadView
2015-06-18 10:12:29.395 JeuDePiste[2412:694523] QCAR OpenGL flag: 2
2015-06-18 10:12:29.395 JeuDePiste[2412:694523] [EAGLContext setCurrentContext:context];
2015-06-18 10:12:29.395 JeuDePiste[2412:694523] ARVC: viewDidLoad
2015-06-18 10:12:29.588 JeuDePiste[2412:694523] QCARutils onCreate()
2015-06-18 10:12:29.588 JeuDePiste[2412:694523] APPSTATUS_INIT_APP
2015-06-18 10:12:29.589 JeuDePiste[2412:694523] APPSTATUS_INIT_QCAR
2015-06-18 10:12:29.589 JeuDePiste[2412:694523] initQCARParameters start
2015-06-18 10:12:29.589 JeuDePiste[2412:694523] initQCARParameters end
2015-06-18 10:12:29.618 JeuDePiste[2412:694523] autofocus: 1, autofocusContinuous: 1, torch 1
2015-06-18 10:12:29.619 JeuDePiste[2412:694523] ARParentVC: appearing
2015-06-18 10:12:29.619 JeuDePiste[2412:694523] ARVC: Rotating to Portrait
2015-06-18 10:12:29.620 JeuDePiste[2412:694523] ARVC: viewWillAppear
2015-06-18 10:12:29.620 JeuDePiste[2412:694523] ARVC: viewWillAppear
2015-06-18 10:12:29.671 JeuDePiste[2412:694981] INFO/AR(2412) 2015-06-18 10:12:29: QCAR SDK version 4.2.3
2015-06-18 10:12:29.687 JeuDePiste[2412:694523] EAGLView: layoutSubviews
2015-06-18 10:12:29.688 JeuDePiste[2412:694523] EAGLView: layoutSubviews
2015-06-18 10:12:29.694 JeuDePiste[2412:694523] ARVC: viewDidAppear
2015-06-18 10:12:29.694 JeuDePiste[2412:694523] QCARutils onResume()
2015-06-18 10:12:29.694 JeuDePiste[2412:694523] initQCARParameters start
2015-06-18 10:12:29.695 JeuDePiste[2412:694523] initQCARParameters end
2015-06-18 10:12:29.695 JeuDePiste[2412:694523] ARParentVC: appeared
2015-06-18 10:12:29.695 JeuDePiste[2412:694523] ARVC: viewDidAppear
2015-06-18 10:12:29.729 JeuDePiste[2412:694981] ERROR/AR(2412) 2015-06-18 10:12:29: Invalid Key used. Please make sure you are using a valid Vuforia App Key
Thank you for your help!