Weird glitch: Tabs disappearing, page title becomes "Navigation"...

I've been setting up a Drupal 5.7 site for a while now, and I've almost settled on the right combination of modules I want to use. Today I logged in to do some more work, and found that something very weird was happening: On any page that includes tabs, when I go to any tab other than the first one, the tab bar disappears entirely, the breadcrumbs are reduced to just "Home >", and the page title (<title>, <h1>, and everywhere else) is changed to "Navigation"! This last effect doesn't happen everywhere, but in places where it doesn't (user pages, views, and maybe elsewhere), the page title is instead changed to the title of the first page in the tab group.

First I might think to go over the changes I've made recently. Big problem: Yesterday I made so many changes I can't even remember them all! Installing and uninstalling modules, adding and deleting content types, fields, templates, views, and actual content... about the only thing I didn't change was anything to do with the theme (Zen), which would appear to rule out the easiest way to start the investigation. Here are the modules I currently have installed:

ACL + Content Access
LDAP Integration
CCK
Content Templates
Node Hierarchy
Date API + Calendar
IMCE
Pathauto
Theme settings API
Token
Update status
Views

All of these were previously in place and operational before the problem occurred, except for IMCE which I installed after it started happening. I was also previously fiddling around with imagecache, imagefield, and imagefield gallery, but I have since disabled and uninstalled them.

I honestly have no idea how to get started debugging this kind of problem; there are no unusual messages or errors of any kind that could at least tell me where to start looking. How do I find out what's going on here?