The page is getting a javascript error about not being able to find a specific panel when you click on the edit html button:

That panel seems to be inside commented-out code:

Probably due to the fact that the content component block directly following this HTML block is trying to comment-out it's output by adding HTML comment elements in the pre/post advanced setting.

When you remove those comment elements from the content component block, the HTML block's edit dialog box works correctly.