ModelCategory not found in file

I'm having problems unpublishing from within the EasyDiscuss admin area -

1) If I display the category list and then click on the check button in the "Published" column to unpublish any particular category, nothing happens.

2) After selecting a specific category from the category list and then selecting the "No" (unpublish) toggle and then saving that selection, I get the following error message:

Model class easydiscussModelCategory not found in file

Any help appreciated.

Thanks. -- Rik