I recently started receiving this "Unable to preview the survey" error when trying to load the preview of any survey with an image in a note in the Web Designer. This workflow worked fine up until about a month ago, but now gives an error. When I publish the survey, the image link is broken and the following the link goes to a ESRI 404 page.
Honestly, I'd contact support.
If things were working and then suddenly stopped, it is likely an update they published that changed how things are parsed. It may be that what you were doing previously was incorrect but not caught by the parser, and now it's more strict. Or it may be a bug that needs fixing (Edit: I'm leaning towards a bug, based on your screen shot. I was just listing the two most likely reasons for a sudden change).