Blank panels after upgrade from 6.x-3.5 to 6.x-3.9

I'm trying to update the code base for one of our sites to 6.x-3.9 from 6.x-3.5. The install goes just fine, and I can see all 7 of my pages in admin/build/panels, but when I try to view any of the panels, I get a blank page.

My upgrade goes something like this (if it's important, I'm upgrading via CVS, straight from the Drupal repository):

cd {DocumentRoot}/sites/all/modules/ctools
cvs update -r DRUPAL-6--1-8
cd {DocumentRoot}/sites/all/modules/panels
cvs update -r DRUPAL-6--3-9

http://www.mysite.org/install.php

install.php runs fine; admin/reports/status shows no problems.

Modules I've enabled: Chaos Tools, Page Manager, Views content panes, Panels (Mini panels and Panel nodes are disabled).

If it helps, I've done some crude traces and gotten this far (in panels.module):

  function render($renderer = NULL) {
    $layout = panels_get_layout($this->layout);
    if (!$layout) {
      return NULL;
    }

    // If we were not given a renderer object, load it.
    if (!is_object($renderer)) {
      // If the renderer was not specified, default to $this->renderer
      // which is either standard or was already set for us.
      $renderer = panels_get_renderer_handler(!empty($renderer) ? $renderer : $this->renderer, $this);
      if (!$renderer) {
        return NULL;
      }
    }

panels_get_renderer_handler is getting passed a value of "standard" and returning NULL.

This is a site that I didn't develop and with which I'm not very familiar (and I'm not very familiar with panels either), so any help on how to troubleshoot this would be very helpful! At this point I'm kind of lost in the panels/ctools code.