Same JS error for me too using 5.5.13. But I am using a Flexible Content field. And the issue seems to any layout with a clone field.
I get the error when trying to “add new layout” in the custom fields editor. I get the same error when using the layout button to add a layout to a page if it’s any of my layouts with the clone field.
UPDATE: I tried rolling back to 5.5.11, but it still doesn’t work. Which is weird because it did work at some point, I had the layout added to my page that I can no longer add.
NOTE: Download link for 5.5.12 looks to be actually v5.5.13
