Any idea what’s going on in the attached screenshot? I rendered the image and it was fine. Then I did a region render on the upper half of the image and after the region render finished, the lower half turned black & red. I should note that the red parts are windows with glass material which have Object IDs set from 0 to 1 and I have a MultiMatteElement render element that I use to easily select these parts of the image afterwards. This might be related to the issue? Also, when I disable the Denoiser and Lens Effects layers, the image goes back to looking normal.
Hi,
I encountered similar things and as you mentioned it had to do with the denoiser. I could explain it to me as following (but I have no idea if it IS actually what is happening):
Since you are doing a region rendering the information for the denoising is only recalculated for the region. But since the denoising is applied to the whole image it needs to pull that from previous full size renderings where you didn’t do only regions.
In the meantime probably your scene setup changed, you possibly added, changed or removed renderlayers or had different changes that make the scene back then not the same setup like you have now when you are doing the regions. This way the denoising gets confused and you might end up with those kind of effects.
The solution would be to do a full rendering again to overwrite all the old information in the denoising elements and then continue with the regions.
As said no idea if that is technically accurate, I’m just guessing basically… 
I had that once and turning "calculate denosier "off and on under denoiser solved it iirc ,but disable the region before that .