Strange issue. I upgraded to 3.0 the other day (also running WP 3.5.2) and, whenever The Events Calendar is active I can no longer use the Link to Existing Content feature in the WP Post Editor. It won't even give me a list of the content on the site. Just a message that says "no matches found".
I'm also running the latest version of TinyMCE Advanced. I wouldn't think there would be a conflict there but I wanted to mention it just in case.