I saw this issue on Github and just changed the fallback image, so that’s solved.
This seems to be a bug, but I played around with the inspector and resized my browser window and this appears to just be happening when the browser size has a very specific value, so this doesn’t seem to be important.
However, I’m still stuck with the customizer not working. Any help would be greatly appreciated. You can find the credentials above to log into the site in question: