Registered users can't check out-- checkout_shipping loops back to shopping cart

Hi,

Yes, v1.38a (contractors, get in touch! I need help!)

Just as I mentioned-- new users can go through and place an order without a hitch; registered users get stuck in a loop between checkout_shipping and shopping cart. In the url, you can see the url go from checkout_shipping to checkout_payment, but instead of loading the page, it then goes back to shopping cart.

Any ideas?

Very Important Disclaimer: the client tried to auto-upgrade, dumbly, the other day, then immediately called me when all hell broke loose. I called our hosting provider and reverted to a recent backup. I've had this problem since.

The site is ageing and I don't have time or skills to upgrade, though the client is finally willing to pay for it (losing sales on this one). In the meantime, though, I need to get this resolved. I've tested UPS XML and SEO modules (disabled them, no change, re-enabled, no change) and checked out tables in the db which look like they always have. SSL looks fine. Any immediate tips or places to look would be helpful.