I began experiencing “unable to export” after upgrading to RW8. I have two very large sites, and the option of rebuilding them was unattractive, so I kept hoping to find a solution. I discovered a solution yesterday, which might be helpful to others. I published each page, one at a time, until I ran into the “unable to export” message. I discovered that there were two pages that I could not export/publish, so I deleted and recreated both (one styled text, and one slideshow). Now the entire site exports and publishes seamlessly.
How’s that for a low-tech solution?