“new” and “edit” pages blank after upgrade to 2.7.1

The upgrade from 2.5 to 2.7.1 went ok, apparently, except for one huge thing:

When trying to EDIT a post or page; or to create a NEW post or page, the window is completely blank. The blog appears ok, and the dashboard seems to work fine. The listing of pages and posts look ok, and the plugins seem to work.

But if I click “edit” for an existing page or post, or try to create a new one, the screen is blank. By viewing the source code, I can see that a lot is there, it just doesn’t display on the screen.

I’m using IE8. Tried compatability mode with the same result. Also tried Firefox with the same result. There must be a corrupted config file somewhere.

I also tried updating (or resetting?) the permalinks with no result.

Thanks in advance for any ideas…