Sorry, was reading on my phone. Hard to see the whole message sometimes.
I looked at both saves, and you've got me. I started with the save where you could build US, and went all the way up to the same date in the save where you can't build it, and I could build it just fine. Something must have happened between the two, but there's nothing I can think of that would effect the building of that wonder empire-wide. What's even stranger is that I played through to 1530, and suddenly I can build it. 1525, I couldn't, but a turn later, I could. I didn't see anything that could have changed it during the play up to 1530, and I didn't make any significant changes to the game as I went.
Are you using a mod of some kind? I ask because the font for the cities on your F1 screen is different than usual. It's italicized.
Edit: Between 1425 and 1525 is exactly 20 turns. Can anyone thing if something that lasts 20 turns that would prevent building a wonder?