when i try to adjust my theme's appearance, it tells me that my session expired

i am currently running WP4.1, but back when i was upgrading to WP3.8, i had to change themes, because my old theme broke when i upgraded to WP3.8. the theme i had been 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 when it was current (WP3.8), 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.

this has been a problem ever since WP3.8, and it has had the same result in every WP version that i have tried. i have tried this with all my plugins deactivated and it makes no difference. nobody responded to my previous post, and it is now closed to new responses, so this is a new, updated post. will anyone respond to this one?

my site is at https://przxqgl.hybridelephant.com/