I'm having the same problem..
I just got my M8 today, played with it for a couple of hours before unlocking the bootloader. Restored titanium backup after unlocking, everything was fine until I opened the camera. I keep getting the "duo camera blocked" warning, I tried taking pictures for an hour straight but the warning persists and I can't use U-focus or any other duo feature on every picture I take.
I'm on stock ROM and recovery and unrooted. Tried to restore factory settings, no success. Just updated to the newest firmware (1.54.401.10) via OTA but still no working duo camera.
Did you find a fix for it or did you send it back for repairs? I'm a little frustrated because I unlocked the phone before noticing the camera problem...