I never really figured out what the error message meant. I did finally get it working, if I remember correctly, I think this is what happened.
As I mentioned in my original post, I have several spaceship blueprints with similar Niagara particle exhaust systems attached. Since they are all similar, I created them by copying and pasting from one blueprint to another. I am typically rabid about debugging and figuring out what is wrong and correcting. But in this case I became exasperated and just deleted the broken blueprint and built it again completely from scratch. Guess what, it worked.
Sorry, that may not be much help, but it’s all I got.