Logon as administrator (or any other user with administrator access and enabled account protection). You should be able to change some users settings - for example language, but after clicking 'Update Prefs' button on page manage_user_edit_page.php you get error message 'ERROR: This account is protected. You are not allowed to access this until the account protection is lifted. (user_api.php: line 211)' on page account_prefs_update.php. It is caused by current_user_ensure_unprotected() function which is called with no clear reason (it's not current account being changed). I think that user_ensure_unprotected() with selected users' id should be called after reciving form data.