This thread was closed, but I feel like I have some information to share. I’m at 8.1.6 and this happened to me this morning. I have no clue why, but it occurred on one project and not another.
I opened up a new project and dragged every page into the new project, copied the code page(s) and publishing info and everything is back up and running in about 10 minutes.
Hope this helps anyone else stuck on this error trying to preview their site within RW.