When I edit a user, all the changes get saved properly but I receive the following error:
Your attempt to edit this user: "" has failed.
It appears that WordPress has lost the user name after saving but before reloading the page.
One piece of background information: I restored a WordPress database backup from PhpMyAdmin, and I changed the database prefix (with find-and-replace in a text editor). I do not know if it worked before I restored the database or not.