Hmmm......just might be a faulty infrared sensor. Can't help you specifically with the Stealthcam; I've only ever had Moultries or Cuddybacks. Is there anyone close by that could loan you a camera that's known to take night images, to use as a control? Might also be a good idea to call the manufacturer & explain the problem to their customer service rep; might be a simple fix. If push comes to shove and you want to replace it, you might be able to catch one at a reduced price, since hunting season is essentially over, or soon will be. There's not an option to disable the infrared, I assume, like a switch somewhere on the front that could have inadvertently been switched off.
I wish I could give you more in-depth advice, but I'm fresh out of ideas.
If you think of it, I'd be curious to know the solution. Good luck.