Thank you, Cindy.

No other logs were thrown that I would say are associated with checkout. We do get a few related to duplicate language constants, but I don't think that has anything to do with the issue described.

I've installed Report All Errors and will start watching it now, maybe that will take me somewhere. I'll report back when there's some data.
As for how "buggy" it is - I built that site so... uhm... okay, I'm ready for a huge log file...

It's ZC 1.5.6. Period. No sub-version, just 1.5.6