I had to get it working, so started rebuilding pages. I started by rebuilding the page with the ATC, and only in steps. I found that when I added Server Validation, the form would work once, but if I clicked continue shopping it would no longer post to the cart. That included if I opened the page in another browser, and started from scratch.
I'm not sure if I'm doing something wrong in the validation behavior, but I'm not doing anything differently than I have done before.