An unrecoverable error occurred. This form was missing from the server cache. Try reloading the page and submitting again.

When trying to upload a file, Drupal shows the message:
An unrecoverable error occurred. This form was missing from the server cache. Try reloading the page and submitting again.

Same issue reported by #46 and #56 in http://drupal.org/node/500646#comment-3249114

Disabling the popup module fixes the problem. This happens only in one of my Drupal sites (test and production) and not on another with similar setup.

I'm using Drupal 6.20, popup 6.x-31, CKEditor - WYSIWYG HTML editor 6.x-1.2, Views 6.x-2.12