I've been trying to solve this and it looks like CCK version 6.x-3.x-dev is also having people report the same issue - http://drupal.org/node/541768

All was working OK until I updated a display field on one of my content types by changing the label from "Above" to "Inline".

Since then, the following warning message is being displayed for that content type:

warning: Illegal offset type in isset or empty in .../sites/all/modules/cck/content.module on line 1380.

Drupal 6.22
PHP 5.3.2
MySQL 5.1.41
CCK 6.x-2.9

Can anyone help?