I get this message on the homepage only.
The preview could not load. This is most often related to a plugin conflict or aggressive page caching. Checking the developer console for errors could indicate what went wrong.
I followed all the suggestions and then made a new homepage, called home. The original homepage was auctions; I made the new page called home and then when I tried to edit it, I could not it came up with the same error above. But now if I try and edit the auctions page, it allows me to edit it.