Redirect to Whoops! after post

Whenever I publish a post, I am redirected to the page that says “Whoops! Whatever you were looking for cannot be found.” I believe this is a 404 error…? The post gets published.

In my error logs, I get this:

[Sun Nov 21 22:43:20 2010] [error] [client] File does not exist: /home/tastethe/public_html/wp-content/themes/on-assignment3/images/li.png, referer:

WP 3.01

When I delete a post, I also go to the “Whoops!” page. The URL reads:

Do you have any idea what could be causing this? Today I updated Communities, Easy Blogging, Update Notifications, Avatars, and attempted to update Messaging, but the update caused a conflict, so I deleted it. Could one of these updates have caused the problem?

Thank you!