i've been using WP for years and this is the first real problem i've had with it.
i recently had to change themes, because my old theme broke when i upgraded to WP3.8... the theme i was using hadn't been updated since WP2.6, and so it wasn't a big surprise, but i quickly discovered that changing themes wasn't as easy as i suspected.
i switched to the Twenty Fourteen theme, because i couldn't find anything else that works at all, but when i try to adjust the appearance, i go to Admin -> Appearance -> Themes, select a theme and click "Customize" and the following happens:
1) i see a two panel window, with information about the different options in the left panel, and a wordpress login screen in the right panel. the login screen has an alert box with a green stripe in it that says:
Session expired. Please log in again. You will not move away from this page.
2) when i enter my username and password, and click "Log In" i see a screen that says "You have logged in successfully" which disappears and is replaced by a duplicate of the wordpress login screen seen in step 1.
i can never move beyond this, and thus can't adjust any of my theme's options, regardless of what theme i am using.
i have tried this with all my plugins deactivated and it makes no difference. i have also noticed that a number of other people have had similar problems for approximately 1 year, so i'm hoping that there has been some action to fix this issue.
my site is at http://przxqgl.hybridelephant.com/