Theme customizer preview screen is broken

My theme customizer preview screen is suddenly broken. A large part of the preview screen is not visible. It is grey and inactive.
See snippit here:
https://i.stack.imgur.com/YOInq.png

I have tried the following.

– Tried disabling all plugins. Did not work.
– Tried switching theme. Did not work.
– Tried restoring an earlier backup, where the the theme customizer worked. Did not work.
– Tried reinstalling wordpress. Did not work.

Could it have something to do with the core files (eg. functions.php)?

The page I need help with: [log in to see the link]