If you receive this message, go to step 0 and select "Preference". The option "Project folder" shows you the path in which your  WebSite X5 Evolution 9 project has been saved.

Go in this folder and proceed as follows:

1. Delete the "preview" and "upload" folder.

2. Delete also the files  project.iwprj and metadata.xml.

3. Rename the file backup.iwprj into project.iwprj.

4. Open the program and select it from your recent  projects.