I thought it might be a resource problem as well. I added a blank page, deleted all other pages, deleted resources one file folder at a time until only the added blank page was left. Did one final save, closed Rapidweaver, reopen with no change.
Started a new project, added all the same resources, added the pages one at a time until the project was identical to the original. Problem solved.
This is something that was added by Realmac and by opening, then saving the project hopefully you got a backup as you can not open a 8.1 project using 8.03.