I had the exact same problem with the categories.

Slackware Linux-current kernel 2.4.19, PHP 4.3.0, Apache 1.3.27, mySQL 3.23.54

When I checked the contents of the bb_categories table, all of the category names were null.

Turning register_globals On instantly fixed the problem.

register_globals is on.

Added .htaccess file with "php_value register_globals 1" line in the root directory of my site and in the admin folder of the newbb folder.

Neither worked. I'm still getting the error:

"You did not fill out all the parts of the form.
Did you assign at least one moderator? Please go back and correct the form."

And I DID create a category first, and DID select multiple users as moderators.....

This seems to be a wide spread problem and i haven't found a definitive solution anywhere on the boards.