Oopes! You forgot to selecta category

After I have set Default Post Category of wordpress 3.0.1 correctly, I found I can not post posts successfully, the wordpress show: Oopes! You forgot to selecta category!

How come, I am sure I have set Default Post Category already! I googled for half an hour, still can not find an answer. So I try to disable the plugins one by one, at last I can post posts after I disable the plublin “Article Directory 1.6.1 by Dimox”.