I just tried it on my setup (FEC 1.80 and Zen-Cart 1.3.9b) and it worked as it should. I left off a field on the new address and it came back with an error message and the address page for me to correct it. When I corrected it, it went forward as it should. I am setup as a single page checkout.
Did you just install the FEC 180 or merge files?
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