Upload manager doesn’t appear

I upgraded from WordPress 2.0.7 to WordPress 2.1.

Now I have the problem that the upload manager does not show up when I write a new post or page. It shows the message “This property must have support for iframe.”

I upload fresh 2.1 files again, overwrite the files and click for writing a post or a page and the problem seems resolved.

For it’s first time it is. After publishing the post, when I want to write another post or page for the second time the upload manager doesn’t show up and the problem occurs again.

Why is this happening ?? Can anyone please help..