Are you using a language other than English?
That's a known issue with some older themes. The easiest way to fix it is to replace the theme's "admin" directory with the admin directory from one of the default themes. Since most custom themes didn't change anything in there anyway, this shouldn't make a difference for the theme, but will make it work again for you.
If you're interested in the details, please see the
documentation.
bye, Dirk