Can’t write wordpress post, can’t save draft, can’t deactivate plugins

I am using 2.8.2.
Suddenly, I got strange problem today.

When I tried to post new one. It just give me blank result and bring me to edit post view.
Usually, before I can publish post/save a draft, the post permalink will be displayed. But in this case, it is blank.

I tried to deactivate some plugins. Strangely, after clicking “deactivate” button, I got ” Plugin deactivated” at the top, but when I check back the plugin status under it , it is still active. As a result I can’t deactivate all my plugins.

I tried to delete wp-content/cache, but it has no effect.
I tried to use PHP version 4 ( from version 5 ), but it has no effect.

How can I solve this? I noticed some people using 2.9.1 also experienced it, right?

Regards