Customize Screen Loop – Unable to Authenticate

I’m in the process of trying out new themes, and I think I may have disrupted something in the authentication process.

When I log into my site and go to Appearance -> Customize, it begins to load the customization screen and gives me the left sidebar, but it doesn’t show the site. Instead, it asks me to log in, saying “Session expired. Please log in again. You will not move away from this page.” When I log in, it tells me I’m successful, but then the log in screen loads again with the same message (“Session expired. Please log in again. You will not move away from this page.”) Some more facts:

– This happens for both my admin logins.
– I’ve updated to 4.2.1 and then reinstalled via the update page.
– I’ve got another site that works fine, and it has also been updated to 4.2.1.

I think the issue is that I changed my theme three or four times and even though I’ve changed back to 2015 it won’t let me customize (that’s right, I can still change the theme and it changes on the main page).

I would really appreciate some help, and quick. I’ve just started up my blog and am trying to get readership!