Malfunctioning modules page on Windows XP after upgrade from 1.9 to 2.x.dev

This is probably related to #310873: D6 Upgrade path broken when CCK modules are disabled → .

I'm trying to upgrade from Drupal 5.10 CCK 1.9 to Drupal 6.4 CCK 2.x dev.

• Drupal Core upgrade: OK
• CCK Content upgrade: OK
• CCK Fieldset upgrade: OK
• CCK Number upgrade: OK
• CCK Text upgrade: OK
• CCK Option Widgets upgrade: Fails

After enabling CCK Option Widgets I go to update.php as per usual. The upgrade script runs and throws no error.

1. From the upgrade page I try to return to /?q=admin, get a Page not found.
2. At /?q=admin/build/modules I get the modules list, but none of my enabled modules are checked.
3. The sidebars on the modules page are missing. (Elsewhere on the site I still have them.)
4. The Admin Menu module no longer loads.
5. The whole site is excruciatingly slow.
6. No errors or warnings in the logs.

Disabling the Option Widgets module in itself does not solve the problem. I get my modules page back only when I disable Content as well.

I've run the upgrade 3 times: twice with RC8 and once with 2.x.dev, with the same sad results.