After upgrading Drupal from 4.7.3 to 5.1, and Category module to recent 5.x-1.1, my containers show correct summary of categories with assigned nodes count, but all the category pages are blank (no nodes listed), plus all the categories and containers edit-forms break (if I click the "Edit" tab, I get only heading, plus error in watchdog, about bad callback argument or whatever, not experienced in this).
Thanks to some hints in this forum, I found a solution finally:
All the mess is about Category content-types being renamed lately - from old "category-cat" and "category-cont" to new "category_cat" and "category_cont". There are a few occurences in the code being not updated, and for mysterious reasons the database update script (Category module update #9) didn't work - even copying the relevant queries into PhpMyAdmin didn't! - leaving me with old database schema too. All the problems originated from mismatching the old and new names in the system.
The fix:
--- Search all the files under the Category directory, excepting .install and .info, for occurences of "category-c" and replace with "category_c". If you have the Book- and Taxonomy-wrappers installed, uninstall and install again, to update the files.