I'm unable to delete a current project and unable to create a new one, instead I get an error that says: The operation could not be completed. No other information is available about the problem. I'm currently using iMovie with the Mavericks system.
It might be a permissions issue - I would check out the troubleshooting steps in the following document and see if that helps:
Troubleshooting permissions issues in Mac OS X
I have the exact same problem. Deleted and reloaded imovie, but that didn't help.
Retrieving data ...