It seems there were a few faulty Nuke scripts that were the problem. The red X would pop when viewing from some nodes in the stream and not others. There were 2 glow nodes at the end of the stream before the write node. One of the glows would produce the red X and the other wouldn’t. Even more strange, the write node wouldn’t produce the X but the glow before it would.
I am still getting to the bottom of it, but all signs point to a buggy Nuke script. No need to log it to tech supprot.