My project has an NGUI overlay that works fine on iOS, PC play mode, and sometimes on Android. A majority of the time, however, the AR camera video background is black when the app starts on Android Nexus 4 (4.4.4) and Nexus 5. Unity version 4.5.4f1, Vuforia 3.0.9. Seems to work fine on a Nexus 10 tablet. I'll try some other phones soon.
When I rotate the phone the video sometimes turns back on, but then will turn off when I turn on / off other cameras, too. Leaving the app and returning also can change the state of the AR camera from working video to black, or the other way.
When I turn off the NGUI camera everything seems to work.
There are lots of other threads talking about a black video background, but I believe most of these refer to bugs (alpha rendering, 32bit / 24bit buffers) that were fixed in Unity already.
I've tried unchecking 32 and 24bit display buffer in the Player Settings, no luck. Nothing looks strange in the logcat of QCAR
I'm still trying to recreate the problem building from a standard vuforia tutorial. In the meantime, any tips for debugging or working around this issue?