I just wanted to let you know that using a fresh install (from the Drupal release, not the zip file you posted) with only the modules loaded that Ubercart needs I got the user portion to work. I'll continue to tweak the new version and see if some other module conflicts with the settings again. Just for your information, on my main site these are all the modules I had loaded, even though many had been disabled and uninstalled if possible. I'm guessing one of them stepped on your $_SESSION variables, or on the conditions under which they are set.

bbcode, bio, captcha, captcha_pack, cck, cck_address, cck_address_extensions, cck_button, devel, devel_forminspect, dhtml_menu, email, filter_default, forum_access, imce, link, lm_paypal, nf_registration_mod, nodefamily, nodeprofile, pageroute, pathauto, phorum, quicktags, retease, site_map, smileys, subform_element, tapir, tinymce, token, ubercart, ubrowser, usernode, views, views_bonus, views_fusion, workflow_ng

Please let me know if any of those concern you or are known issues. My money is on lm_paypal, which I had deactivated (had no uninstall) and will not be reinstalling, just because it too was also doing role purchases and seems a likely target because much of what it does doesn't conform to Drupal standards. I'll keep you informed if I find out more. Thanks for your time and help and for your overall efforts on ubercart.