Think I've figured it out. I followed Thomas' suggestion, in part. I deleted all of the flight plans that I created,
including the ones in question, from the Flight Simulator X files folder. All that I kept were the example/tutorial ones
that came with some of the add-on airplanes.
I tried to load some of those, and to my surprise, they worked. Using FS Commander, I created a brand
new fresh flight plan of my own.
Sure enough, PF3 accepted and loaded it without a problem.
Don't know what exactly happened, but it looks like I'm back in business.
