All users but user/1 denied access to all content

I'm pretty sure this is not a bug but a glitch in my database somewhere. I've had a smooth upgrade, and the site is shaping up ... but only for my user/1 admin account. What I am seeing otherwise:

1. All other users are denied access to all content. This is even though I have rechecked the access settings 100 times. Access content is enabled for all users.
2. Experimenting, I discovered that all other users are denied access to even front_page. This is even though I have enabled front_page access. Turning the module off again does nothing. The users simply cannot access /node or any node content, new or legacy.
3. All other users see the default Drupal welcome message -- "TWelcome to your new Drupal-powered website. This message will guide you through your first steps with Drupal, and will disappear once you have posted your first piece of content...." -- even though there are a dozen or so existing posts. I created a new test post, promoted it, and still all other users get that initial welcome message.
4. I created a new user (user/9) and still ran into all the above problems.
5. Changing themes does not affect any of this.
6. Interestingly, user profiles are accessible.
7. Non-clean urls (e.g., ?q=node) are not found. Not surprising, as this is an access denied problem.

Okay, here's the backround I think might be relevant. My proper backup with all the extra modules disabled ended up getting corrupted, so I had to insert a week-old database backup (with modules not disabled) to use for upgrading. One of those modules was taxonomy_access. I'm suspecting that there is a lingering permission setting that I cannot access now with the current system. I dropped the term_access table, but that did not fix things. I cleared the cache, but that did not fix things.

If I am dealing with a legacy setting from the old taxonomy_access module, is there another place I should look, another table I should drop or a line I should edit? Any clues would be appreciated!!!