Update in case anyone's interested...
It must have been a weird bug. What fixed it for me - After trying several other roms the problem still persisted. Eventually I installed a stock Google Play rom and updated via OTA. I cleared the camera cache (which I did on the several other previous roms), this time it worked!
I restored ViperOne, and all's good.
I did notice quite a few posts scattered around the web that were asking about this, especially for the HTC One LTE. I don't know if it's the same issue, but a lot of folks seemed to think it was a hardware issue.