Stuck in "Session expired." when trying to customize a theme

(edited from original post)

WordPress 4.2.2
Theme is now Twenty Fifteen
All plug-ins are disabled.

Not just an hour ago, I was logged in on two browsers (Chrome and FF). After sorting out other problems, I ran into this one.

Whenever trying to customize a theme, I get:
“Session expired. Please log in again. You will not move away from this page.”

I’ve tried removing the www from the site name, as mentioned in 2-3 year old threads; no effect. I’ve tried every theme I have installed, including all these official wordpress ones: 2010, 2012-2015. Live previews also don’t work, same result.

Everything on Chrome is set to recommended, I even added my site to always allow, no change.

I tried logging in with FF to see if it was just Chrome, but it doesn’t matter. I’ve checked, and I’m good on cookies: https://en.support.wordpress.com/third-party-cookies/

(Because editing was no longer allowed)

And now I’ve also tried adding WP_HOME and WP_SITEURL to the wp-config.php, but that messes up the admin dashboard even more, and it’s impossible to tell if customize works.

My wordpress home is at mydomain.com/parkeddomain.com/wordpress

And the site url is https://www.parkeddomain.com.

None of the move documentation addresses this at all. They all assume mydomain.com/…/wordpress is going to be for a website at (www.)mydomain.com.