I recently upgraded to 2.8.4 and have discovered that many of the boxes on the Add New Post page in the admin area are not displaying. Assuming you count nav as a column, everything after the trackback box in the middle column doesn't appear. Boxes that are missing include, but are not limited to: custom fields, discussion author, and twitter (via plugin). If I view the source I can see that the page stops renderings or stops downloading after the trackback box since it stops there with no ending html tags.
I have already tried disabling all plugins; that did not solve the problem. All of the boxes I mentioned appear on the edit post page, but not the add new post page.