To be clear, the denoise node has been working wonders for me, but sometime over the last week or so it has stopped working in all of my project files (including ones that worked).
I have a single denoise node connected between the render layer and the composite and viewer outputs. The image is shown if I remove the denoise node. No errors are produced in the console during the rendering or compositing.
I wonder if this could be a file permission issue. I'm running linux, and when I started using Luxcore Rendere, it's denoiser couldn't run because the denoise program wasn't marked as executable. Doing so fixed the issue with luxcore.