I have this exact same issue on a site I’m working on but upgrading didn’t work in fixing it.
It seems that when the Edit button gets clicked, it’s trying to the URL for the site instead of the data it needs along with the input field. At least, that’s what it shows in the Console.
When I try to update the Thumbnail, it opens up the modal but instead of showing the regular thumbnail chooser stuff from WordPress, it shows the Home page for the website inside. Very strange.
Any help in trying to fix this issue is greatly appreciated.
