I have this exact problem when I add a Calendly popup on the page via the code block.
I can edit header and footer only, but the rest of the page doesn't show the drag'n drop lines. After logging out and back in, or clearing cache several times, I can finally edit other blocks, but when I try and move them they all go to the top of the page and I can't place them anywhere else.
Any idea how to fix this?