Fatal error: Call to undefined function: mb_substr() in /wp-admin/includes/post.php on line 955
This error appears directly below the post title where the permalink edit link is supposed to be. This was a clean install of 2.7, upgraded to 2.7.1 prior to writing any posts. The first couple of posts were fine and this suddenly started.
The result is an inability to edit posts because all fields below the error are missing once they are published, yet the quick edit from the main posts page works - allowing changes to the permalink. The error occurs immediately once WordPress does the autosave while you are writing the post, but the fields remain in place - it is only when you go back to edit the post that all fields are missing.
Pages are fine.