I had the same problem on Magento 126.96.36.199. Turns out that the page cannot load correctly because of the translation: a string uses invalid characters.
Try switching the admin language to default English and see if the "Layout updates" section loads correctly. If yes, check the error console: the error that you mentioned will not be present any more.
In this case, switch back to your admin language and look for the following error in console:
"Uncaught SyntaxError: Unexpected identifier"
Examine the referenced row, it contains the invalid string. Edit/remove it from the translation. Clear cache and reload page. When all invalid characters are removed from the translated string(s), the page will load correctly.