1.3.8a 500 error on checkout

[darrellonsite]

When someone continues checkout button on the shipping page (choosing Walk In or shipping) I get a white screen or 500 error. I get no php logs and I've verified PHP logs are working by forcing a basic error. I also tried switching the php/sendmail email settings. I've seen some the old posts from 2006 and they didn't help me.

I've spent an hour stumbling around in files not really knowing what Im looking for. Usually reinstalls can take up to 6 hours depending on the plugins and such (back and forth with client checking every detail). I'd hate to have to upgrade as the site was working fine before and looks like there is a 500 error and other checkout problems for some with 1.3.9f anyways. Its just been copied over. (dont have originals anymore). Client really can afford to spend 6 hours on this. Is there a specific file to check?

The button clicks a POST to this page and goes blank. But If if access this URL directly it loads and tells me to login.
/store/index.php?main_page=checkout_payment