[Theme: Tonic] Weird issue with opening customizer

Hi everyone! I hope everyone’s having good holidays.

I use the free version of Tonic. Two days ago, the theme suddenly developed an issue: when I try to open the customizer for theme options, the menu appears for one second (during which it’s inaccessible) then disappears.

I’ve tried entering the customizer through all three or four different routes, and it’s just inaccessible. It shows me the little paragraph that explains what the customizer does, as if I just started working with this theme.

I’m using the same Admin username I always use. I loaded a couple new plug-ins yesterday (including the pro version of Expanded Media Library) and I disabled them to see if that made a difference, but no luck. And I’ve never worked with child themes or touched the code myself in any way.

My site DID get the new WP update this weekend. Are WP and Tonic interfering with each other? Has anyone had this issue?

My site is https://koreandrama.today, though looking at it might not tell you much.

Many many thanks for any ideas or suggestions!

Odessa